7 ms·

> Pass a cognitive test asking me to do basic arithmetic and convert fractions as fast as I could. I would fail this. I have had a lifelong inability to do sim

> Pass a cognitive test asking me to do basic arithmetic and convert fractions as fast as I could. I would fail this. I have had a lifelong inability to do simple arithmetic mentally. When I had to learn multiplication tables I finally memorized them by using flash cards and a payment schedule set up by my parents. In fourth grade we had "time tests" where you had to do a page of sums and were graded based on how quickly you completed it. I never finished them in the maximum time allotted because I had to count on my fingers. Even today I will sometimes do that, or I will use offsets from a few sums that I do know. For example, 7+6. For some reason I know 7+7 is 14, so 7+6 is one less than that. Hence 13. Anyone else think this way as an adult?

the_af 6y ago
I have trouble with basic arithmetic as well, never learned the multiplication tables and employ all sort of tricks like the one you mention. I still blank out when someone wants me to do basic arithmetic on the spot. It's a bit embarrassing.
- crocsarecool 6y ago
  Oh gosh me too!! I’m so embarrassed to do mental math. Unless I have a pen and paper or use of my fingers, I can’t do it. It feels like the numbers start disappearing from my head as soon as I start calculating.
  - cutemonster 6y ago
    Does this happen also if your're at home alone, and trying some arithmetics?
    - crocsarecool 6y ago
      Yup! If I can write my work down, I’m okay, but I feel like the numbers slip if I calculate in my head. If I’m trying 9+8, I’ll think 9 is close to 10, so I’ll +1, now I can do 10+8=18. As I work this in my head, I’ll start to forget that I originally started with a 9, and I have to figure out what number I used (+1) to make the problem easier, which often makes me restart the problem. It’s like a loop. I can’t do word problems very well either. There are so many numbers, then you have to figure out what they’re really asking. It’s frustrating because I can tell it’s simple stuff, but I just can’t hold on to everything at once. I guess it’s like a learning disability. That said, for every pain I’ve had with math, I’ve breezed through with English and writing. When I was younger doing my SATs, I completely tanked math, but had a perfect score in the English and writing sections.
dblohm7 6y ago
> In fouth grade we had "time tests" where you had to do a page of sums and were graded based on how quickly you completed it. I never finished them in the maximum time allotted because I had to count on my fingers. Even today I will sometimes do that, or I will use offsets from a few sums that I do know. For example, 7+6. For some reason I know 7+7 is 14, so 7+6 is one less than that. Hence 13. Anyone else think this way as an adult? When it comes to adding and subtracting, that's definitely me!
SixDouble5321 6y ago
Memorization of numbers generally, but yes to the above.
fermentation 6y ago
I also had trouble memorizing arithmetic/multiplication tables. To this day I have no idea what 7*8 is (I can spend a few seconds figuring it out but I just never memorized it like most people). For some reason my university math courses disallowed calculators, and I vividly remember sitting in my Calculus III course counting those "number dots"[0] on every exam. [0]: https://64.media.tumblr.com/uwQaNeR4EjfyqysnvQFexD3vo1_400.jpg https://64.media.tumblr.com/uwQaNeR4EjfyqysnvQFexD3vo1_400.j...
- Kluny 6y ago
  Think 5-6-7-8. 7 times 8 is 56. Or use a calculator, who cares. Arithmetic is mostly memorization.
- sneak 6y ago
  I'm in the same boat. 8*7 is naturally 8 squared (64) minus 8. :D
  - Ekaros 6y ago
    Or 7 squared (49) plus 7. Which for some reason is stuck in my brain...
- pavel_lishin 6y ago
  I don't understand how those number dots are supposed to help with multiplication.
- stordoff 6y ago
  I never bothered learning them either (apart from the squares, which stuck quickly for some reason) - I just got fast enough at working them out from known values (squares, 10x is trivial, 5x is half of that) that no one could really tell the difference. I think it actually helped me in some ways, because those "tricks" (mentally breaking it down into simpler steps) generalise to larger numbers quite well.
- ergot_vacation 6y ago
  Exact same thing here. I'm actually having a moment realizing this is more common than I thought. Everyone always acted like it was the easiest thing in the world (even the ones that hated math) and that I must have brain damage or something. But to this day, if you put a gun to my head, I could not tell you what 7*8 was in any less that a minute (and possible not at all without some paper to work it out).
angelbar 6y ago
12+1 for me ;-)
RedEdward71 6y ago
Same here brah. I use finger tricks for much of my quick on-the-spot arithmetic. When I was young(14-24) I could do it in my head, as the years went on I started to rely on some rote memorization and the aforementioned finger shortcuts. These days (in my 50's) I unabashedly use the shortcuts and don't care what wtf anyone thinks...of course the gray in my beard stops most people from razz'in me.
deleted 6y ago
[deleted]
sneak 6y ago
Yes this is how I do the compression of the multiplication table in my head as well. I'm great on the perfect squares, 2, 4, 9, 10 - there are big holes in the rest, and I just add or subtract from values I do know. Fortunately I don't need to use my fingers.
ear7h 6y ago
> I finally memorized them by using flash cards and a payment schedule set up by my parents. This was my exact experience learning to the multiplication tables in 4th grade. Although my payment was watching tv or playing video games. The only one I really learned was multiplying by 8 bc my dad made more effort for me to learn it (he was programmer). I also use random "tricks" for arithmetic, like when multiplying 6 and 7, I'll do (37)+(37) instead because I've memorized 3*7=21 and the addition is easy with no carries
- vagrantJin 6y ago
  I just abhor the fact that teachers never gave students the intuition to approach problems and solve them. Not that I blame them - but my eyes watered when a visiting lecturer derived formulas we'd used in High school and told to memorize as absolute truths without knowing why. You finally get to appreciate mathematics as a discipline and a tool.
  - ghaff 6y ago
    One of the problems, especially but not exclusively at the high school level, is that you often haven't been exposed to the math (or other foundational information) that you need to derive things from more or less first principles. Physics is probably the most obvious example. You need Calculus to derive many of the formulae in high school physics but the average student probably hasn't had Calculus yet so they're just equations to memorize.
    - Wowfunhappy 6y ago
      This is why I think a lot of pre-collegiate curricula is misguided. The ultimate goal of a high school science or math course should be to teach logic and critical thinking ("learning how to learn"). If a student doesn't have the ability to understand why a formula works, we should take a hard look at why we're teaching it, even if that means radically rethinking which subjects are offered.
      - 6y ago
onion2k 6y ago
I honestly hate interview tests like this because you never want a developer to do this in their day to day job. Code needs to be clear and simple to be maintainable, so anywhere a developer has done work in their head and just put in a magic number (eg const foo = 763; // I did the math in my head) you would always reject that in a PR and ask them to show where the number came from. If anything being asked to do mental arthimatic in an interview should be a red flag that the company has a code base that could be full of horrible undocumented nonsense.
- josephg 6y ago
  Yeah; or it’s a sign they just have terrible judgement. “What makes a good programmer? Programming is like math right? Let’s assess that!?” The trick for code readability is to always make the compiler do the math. const time_ms = 30 * 1000; This is much easier to read and maintain.
  - deleted 6y ago
    [deleted]
ergot_vacation 6y ago
It's both relieving and stunning how accurate this is to my own experience. I did TERRIBLE in high school (and college) math, almost entirely because I would constantly make little mistakes in the basic arithmetic. The broader concepts? No problem. But then I'd try to do a problem, slip up a few times on the basic math, and get the wrong answer. And there went the points (and test). I took to calling it "Math Dyslexia," although they're only really similar in that both involved unusual last-mile problems rather than fundamental knowledge or intelligence issues. I suspect you won't reply, this being a throwaway, but was there any ADHD involved? That's what I've gradually settled on for myself, not the "bouncing off the walls" bit, but the bit where you have trouble focusing on anything but the things that are "interesting" to you (and then HYPER focus on those things).
- lamontcg 6y ago
  Sounds a bit more like you may have dyscalculia: https://en.wikipedia.org/wiki/Dyscalculia https://en.wikipedia.org/wiki/Dyscalculia I think what the prior poster was describing is more what I have which I've taken to calling "having shit memory". Can't remember numbers or names, can't learn foreign languages well at all. Had trouble in elementary school with doing multiplication tables fast because I couldn't memorize 7s and 8s, so I'd solve 8x8 by doing 6x6=36 (easy) and then 36+6+6+8+8 to complete the square. I can teach myself optimal control theory, but my speed and accuracy at arithmetic is just getting worse as i can use google as a calculator. I did fine in physics and math courses in college because you can use calculators, and because I always picked the easy questions first, and then ran the answers back through to prove they were right in order to find sign errors and such.
  - cutemonster 6y ago
    What about probability calculations? Like, calculating how likely X is to happen Y times in a row? If you can use calculators Btw from that Wikipedia article, about ADHD: > In 2015, it was established that 11% of children with dyscalculia also have ADHD
    - 6y ago
unanswered 6y ago
> For some reason I know 7+7 is 14, so 7+6 is one less than that. Hence 13. Anyone else think this way as an adult? I also reason 7+6 by going down or up from 14 or 12, but I would not characterize myself as having dyscalcula of any severity.
MaximumYComb 6y ago
I was quite gifted at math, first place out of a school year of 250+ students. I know as an adult that I have all nm for all 0<n,m<10 memorised. When I see something like 79 I use memory and not logic. I feel a lot of math is made easier once some facts become so simple they're memorised. I can do large arithmetic quite easily in my head due to having the basics memorised.
mbrameld 6y ago
I do it by getting to 10 first: 7 + 3 is 10, 6 - the 3 I just added to the 7 is 3, and 10 + that 3 is 13.
schwartzworld 6y ago
I'm not sure you need to memorize the tables to do multiplication in your head. The 10s are a freebie, and the 2s can be done by most people without much effort, for numbers under 100. 5s are just half of the 10s. 9s are just 10x - 1x. 11s are adorable. I'd wager most people internalize little tricks like these for all the multiples of single digit numbers and the lower double digits. Of course, if you do math in your head all the time, eventually you memorize frequent combinations. Anybody who can trivially multiply high doubles digits or 3 digit numbers in their head is exceptional.

the_af 6y ago

I have trouble with basic arithmetic as well, never learned the multiplication tables and employ all sort of tricks like the one you mention. I still blank out when someone wants me to do basic arithmetic on the spot. It's a bit embarrassing.

crocsarecool 6y ago
Oh gosh me too!! I’m so embarrassed to do mental math. Unless I have a pen and paper or use of my fingers, I can’t do it. It feels like the numbers start disappearing from my head as soon as I start calculating.
- cutemonster 6y ago
  Does this happen also if your're at home alone, and trying some arithmetics?
  - crocsarecool 6y ago
    Yup! If I can write my work down, I’m okay, but I feel like the numbers slip if I calculate in my head. If I’m trying 9+8, I’ll think 9 is close to 10, so I’ll +1, now I can do 10+8=18. As I work this in my head, I’ll start to forget that I originally started with a 9, and I have to figure out what number I used (+1) to make the problem easier, which often makes me restart the problem. It’s like a loop. I can’t do word problems very well either. There are so many numbers, then you have to figure out what they’re really asking. It’s frustrating because I can tell it’s simple stuff, but I just can’t hold on to everything at once. I guess it’s like a learning disability. That said, for every pain I’ve had with math, I’ve breezed through with English and writing. When I was younger doing my SATs, I completely tanked math, but had a perfect score in the English and writing sections.

dblohm7 6y ago

> In fouth grade we had "time tests" where you had to do a page of sums and were graded based on how quickly you completed it. I never finished them in the maximum time allotted because I had to count on my fingers. Even today I will sometimes do that, or I will use offsets from a few sums that I do know. For example, 7+6. For some reason I know 7+7 is 14, so 7+6 is one less than that. Hence 13. Anyone else think this way as an adult? When it comes to adding and subtracting, that's definitely me!

SixDouble5321 6y ago

Memorization of numbers generally, but yes to the above.

fermentation 6y ago

I also had trouble memorizing arithmetic/multiplication tables. To this day I have no idea what 7*8 is (I can spend a few seconds figuring it out but I just never memorized it like most people). For some reason my university math courses disallowed calculators, and I vividly remember sitting in my Calculus III course counting those "number dots"[0] on every exam. [0]: https://64.media.tumblr.com/uwQaNeR4EjfyqysnvQFexD3vo1_400.jpg https://64.media.tumblr.com/uwQaNeR4EjfyqysnvQFexD3vo1_400.j...

Kluny 6y ago
Think 5-6-7-8. 7 times 8 is 56. Or use a calculator, who cares. Arithmetic is mostly memorization.
sneak 6y ago
I'm in the same boat. 8*7 is naturally 8 squared (64) minus 8. :D
- Ekaros 6y ago
  Or 7 squared (49) plus 7. Which for some reason is stuck in my brain...
pavel_lishin 6y ago
I don't understand how those number dots are supposed to help with multiplication.
stordoff 6y ago
I never bothered learning them either (apart from the squares, which stuck quickly for some reason) - I just got fast enough at working them out from known values (squares, 10x is trivial, 5x is half of that) that no one could really tell the difference. I think it actually helped me in some ways, because those "tricks" (mentally breaking it down into simpler steps) generalise to larger numbers quite well.
ergot_vacation 6y ago
Exact same thing here. I'm actually having a moment realizing this is more common than I thought. Everyone always acted like it was the easiest thing in the world (even the ones that hated math) and that I must have brain damage or something. But to this day, if you put a gun to my head, I could not tell you what 7*8 was in any less that a minute (and possible not at all without some paper to work it out).

angelbar 6y ago

12+1 for me ;-)

RedEdward71 6y ago

Same here brah. I use finger tricks for much of my quick on-the-spot arithmetic. When I was young(14-24) I could do it in my head, as the years went on I started to rely on some rote memorization and the aforementioned finger shortcuts. These days (in my 50's) I unabashedly use the shortcuts and don't care what wtf anyone thinks...of course the gray in my beard stops most people from razz'in me.

deleted 6y ago

[deleted]

sneak 6y ago

Yes this is how I do the compression of the multiplication table in my head as well. I'm great on the perfect squares, 2, 4, 9, 10 - there are big holes in the rest, and I just add or subtract from values I do know. Fortunately I don't need to use my fingers.

ear7h 6y ago

> I finally memorized them by using flash cards and a payment schedule set up by my parents. This was my exact experience learning to the multiplication tables in 4th grade. Although my payment was watching tv or playing video games. The only one I really learned was multiplying by 8 bc my dad made more effort for me to learn it (he was programmer). I also use random "tricks" for arithmetic, like when multiplying 6 and 7, I'll do (37)+(37) instead because I've memorized 3*7=21 and the addition is easy with no carries

vagrantJin 6y ago
I just abhor the fact that teachers never gave students the intuition to approach problems and solve them. Not that I blame them - but my eyes watered when a visiting lecturer derived formulas we'd used in High school and told to memorize as absolute truths without knowing why. You finally get to appreciate mathematics as a discipline and a tool.
- ghaff 6y ago
  One of the problems, especially but not exclusively at the high school level, is that you often haven't been exposed to the math (or other foundational information) that you need to derive things from more or less first principles. Physics is probably the most obvious example. You need Calculus to derive many of the formulae in high school physics but the average student probably hasn't had Calculus yet so they're just equations to memorize.
  - Wowfunhappy 6y ago
    This is why I think a lot of pre-collegiate curricula is misguided. The ultimate goal of a high school science or math course should be to teach logic and critical thinking ("learning how to learn"). If a student doesn't have the ability to understand why a formula works, we should take a hard look at why we're teaching it, even if that means radically rethinking which subjects are offered.
    - 6y ago

onion2k 6y ago

I honestly hate interview tests like this because you never want a developer to do this in their day to day job. Code needs to be clear and simple to be maintainable, so anywhere a developer has done work in their head and just put in a magic number (eg const foo = 763; // I did the math in my head) you would always reject that in a PR and ask them to show where the number came from. If anything being asked to do mental arthimatic in an interview should be a red flag that the company has a code base that could be full of horrible undocumented nonsense.

josephg 6y ago
Yeah; or it’s a sign they just have terrible judgement. “What makes a good programmer? Programming is like math right? Let’s assess that!?” The trick for code readability is to always make the compiler do the math. const time_ms = 30 * 1000; This is much easier to read and maintain.
- deleted 6y ago
  [deleted]

ergot_vacation 6y ago

It's both relieving and stunning how accurate this is to my own experience. I did TERRIBLE in high school (and college) math, almost entirely because I would constantly make little mistakes in the basic arithmetic. The broader concepts? No problem. But then I'd try to do a problem, slip up a few times on the basic math, and get the wrong answer. And there went the points (and test). I took to calling it "Math Dyslexia," although they're only really similar in that both involved unusual last-mile problems rather than fundamental knowledge or intelligence issues. I suspect you won't reply, this being a throwaway, but was there any ADHD involved? That's what I've gradually settled on for myself, not the "bouncing off the walls" bit, but the bit where you have trouble focusing on anything but the things that are "interesting" to you (and then HYPER focus on those things).

lamontcg 6y ago
Sounds a bit more like you may have dyscalculia: https://en.wikipedia.org/wiki/Dyscalculia https://en.wikipedia.org/wiki/Dyscalculia I think what the prior poster was describing is more what I have which I've taken to calling "having shit memory". Can't remember numbers or names, can't learn foreign languages well at all. Had trouble in elementary school with doing multiplication tables fast because I couldn't memorize 7s and 8s, so I'd solve 8x8 by doing 6x6=36 (easy) and then 36+6+6+8+8 to complete the square. I can teach myself optimal control theory, but my speed and accuracy at arithmetic is just getting worse as i can use google as a calculator. I did fine in physics and math courses in college because you can use calculators, and because I always picked the easy questions first, and then ran the answers back through to prove they were right in order to find sign errors and such.
- cutemonster 6y ago
  What about probability calculations? Like, calculating how likely X is to happen Y times in a row? If you can use calculators Btw from that Wikipedia article, about ADHD: > In 2015, it was established that 11% of children with dyscalculia also have ADHD
  - 6y ago

unanswered 6y ago

> For some reason I know 7+7 is 14, so 7+6 is one less than that. Hence 13. Anyone else think this way as an adult? I also reason 7+6 by going down or up from 14 or 12, but I would not characterize myself as having dyscalcula of any severity.

MaximumYComb 6y ago

I was quite gifted at math, first place out of a school year of 250+ students. I know as an adult that I have all nm for all 0<n,m<10 memorised. When I see something like 79 I use memory and not logic. I feel a lot of math is made easier once some facts become so simple they're memorised. I can do large arithmetic quite easily in my head due to having the basics memorised.

mbrameld 6y ago

I do it by getting to 10 first: 7 + 3 is 10, 6 - the 3 I just added to the 7 is 3, and 10 + that 3 is 13.

schwartzworld 6y ago

I'm not sure you need to memorize the tables to do multiplication in your head. The 10s are a freebie, and the 2s can be done by most people without much effort, for numbers under 100. 5s are just half of the 10s. 9s are just 10x - 1x. 11s are adorable. I'd wager most people internalize little tricks like these for all the multiples of single digit numbers and the lower double digits. Of course, if you do math in your head all the time, eventually you memorize frequent combinations. Anybody who can trivially multiply high doubles digits or 3 digit numbers in their head is exceptional.