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The math behind mind-reading tricks
- kick_in_the_dor 4y agoI came here for magic tricks and was disappointed.
- curiousgal 4y agoWhat is your partner's age? Add two zeros to the end. Minus your birth year from that number. Add the current year to this equation. The result should be yours and your partner's current ages.
- Someone 4y agoNot if the one asked to do that calculation hasn’t had their birthday yet this year (in which case “current year - birth year” doesn’t yield age in years) or their birth year was over 99 years ago (in which case adding “current year - birth year” overflows into the partner’s age)
- neogodless 4y agoClose but not quite. If it helps, both birthdays have not occurred yet this year. If I assume BOTH have, the trick works.
- bombcar 4y agoChange the wording to “you will be this old this year” and it should cover both cases.
- aaaaaaaaaaab 4y agoHere’s a trick I came up with: 1. think of any number! 2. Add your age to the number! 3. Subtract the original number! magical hand motions The result is your age!
- vincnetas 4y agoshow me a kid that can multiply by 13 tree digit number in theyr head and that would be enough magic trick for me :)
- cgriswald 4y agoDefine 'kid.' By middle school, at least, I could have done that even without the (10x + 3x) simplification. The real trick would be finding a three digit number that can be multiplied by 77 and still be a three digit number.
- olddustytrail 4y ago010
- cgriswald 4y agoThat's not a three-digit number. It's a three-digit representation of the two-digit number 10.
- olddustytrail 4y agoThat's not a two-digit number. It's a two-digit representation of the one-digit number A.
- cgriswald 4y agoWell, no, it's not. We're talking in the context of decimal representations of numbers. Outside of such a context the number of digits of a number is meaningless. In any case and in any base, a leading zero would not be considered a 'digit' of a given number, unless the character '0' was used to represent a non-zero digit.
- slingnow 4y agoI don't think any one of these could be classified as a "mind-reading magic trick".
- nestorD 4y agoFun fact, about a hundred years ago, some pseudo-mediums would use a similar kind of math trick to "guess" the number of deceased brother and sisters of someone coming to see them in order to do a Seance. Morality aside, discovering that highly surprised me because I always thought that those techniques were transparent to all people. But I guess math literacy in the US wasn't always great...
- Swizec 4y agoStatistics and probabilities are incredibly unintuitive to people. That’s why it’s called The Birthday Paradox and not the birthday obvious result > The birthday paradox is that, counterintuitively, the probability of a shared birthday exceeds 50% in a group of only 23 people. https://en.m.wikipedia.org/wiki/Birthday_problem https://en.m.wikipedia.org/wiki/Birthday_problem
- criddell 4y agoI’ve never been able to wrap my head around this one. If in a group 23 people at least two share a birthday with a ~50% chance, would that mean if I have a group of 22 people and pick a random day of the year, there’s also a ~50% chance of somebody having a birthday on that day?
- andi999 4y agoNo. The chance would be much smaller. If you choose 22 random days though...
- criddell 4y agoYour comment and erehweb’s finally made it click for me. Thanks.
- erehweb 4y agoNo. There's about a 22/365 chance that somebody has a birthday on that day. But this is getting close to the intuition. Go through the 23 people. For each one of them, there's about a 22/365 somebody else in the group has the same birthday as they do. So adding these probabilities up over all 23, you get 23 * 22/365. Of course, adding up the probabilities is obviously wrong since the events are not independent (and 23 * 22/365 > 1) but it gives the intuition that the probability of a shared birthday grows like N^2. [and actually, you can add up expectations] [Edits for correctness]
- bern4444 4y agoMy own fun math mind reading trick: https://sambernheim.com/blog/the-party-math-trick https://sambernheim.com/blog/the-party-math-trick
- charcircuit 4y agoI was expecting an article talking about probability instead of algebra. People are bad at coming up with random numbers especially if you put some restrictions on what number they are allowed to generate.