3 ms·

I basically just use 6. 1.6 is the factor everyone talks about (approximation of 1.609), but it has 2 significant figures. To make mental calculations easier/q

I basically just use 6. 1.6 is the factor everyone talks about (approximation of 1.609), but it has 2 significant figures. To make mental calculations easier/quicker I use its reciprocal, 0.621, which I approximate with 0.6, which has only one significant figure, 6. Instead of multiplying by 1.6 you would divide by 0.6, which basically amounts to dividing by 6 and then moving the decimal point to someplace plausible. 55 mi / 6 = about 9, so 90 km (actual answer is 88.5) Going the other way: 80 km * 6 = 480, so 48 mi (actual answer is 49.7) If you can't remember whether to multiply or divide, just remember multiplying a number by 0.6 makes it smaller, which you would do if going to miles (which are bigger so there are fewer of them). And dividing by 0.6 makes a number bigger, so you must be going to km (which are smaller so there are more of them).

maxerickson 7y ago
Multiplying by 1.6 is the same as adding 0.6, so the calculation going to KM is about the same either way. For whatever reason, six tenths and add is mentally quicker for me than division.
coldtea 7y ago
>1.6 is the factor everyone talks about (approximation of 1.609), but it has 2 significant figures. To make mental calculations easier/quicker I use its reciprocal, 0.621, which I approximate with 0.6, which has only one significant figure, 6. How about just 1.5 + 0.1? I.e. the same amount plus half, plus a tenth? So, 55 mi -> 55 + (25 + 2.5) + 5.5 -> 88km And inversely, 0.5 + 0.1, so half plus a tenth: 80km -> 40 + 8 -> 48 miles And for most purposes, just (one and a half = 1.5) and (half = 0.5) is close enough. You can mentally add a little more. So 55mi -> 55 + (25 + 2.5) -> 82 (let's say around 85) 80km -> 40miles (let's say around 45 miles)
- JoBrad 7y ago
  This is exactly the way I do math in my head.
- anongraddebt 7y ago
  I often do mental math like this. However, sometimes it is faster to not split up calculations like this. For instance, in the case of converting 55 mi, it's faster (at least, for me) to just divide 54 by 6, move the decimal, and call it a day. I've found that calculations are faster when you know when and when not to split calculations like the way you describe (even though at least 80% of the time it's faster to split them up).
- deleted 7y ago
  [deleted]
BeetleB 7y ago
I had a Canadian car in the US. My rule was simple. I knew the first few multiples of 16 up to 80. And I knew half of 16 is 8. So to convert 35 miles an hour it's just 48+8. And it's very convenient to know that 50 mi is 80km. Beyond that it's just adding 8s or 16s. For going the other way is just find the nearest multiple of 5 in miles and do a quick approximation. So 70 km. I know that 40 mi is 64 km. Thus 45 mi is 72km. So I'll guess a little lower than 44mi
chibg10 7y ago
1.6 = 8/5 So n miles = 8n / 5 km And m km = 5m / 8 miles Fraction multiplication is actually much easier than decimal multiplication imo. I'm surprised so many people seem to avoid it. Keeping track of where the decimal point should be is a significant cognitive load.

maxerickson 7y ago

Multiplying by 1.6 is the same as adding 0.6, so the calculation going to KM is about the same either way. For whatever reason, six tenths and add is mentally quicker for me than division.

coldtea 7y ago

>1.6 is the factor everyone talks about (approximation of 1.609), but it has 2 significant figures. To make mental calculations easier/quicker I use its reciprocal, 0.621, which I approximate with 0.6, which has only one significant figure, 6. How about just 1.5 + 0.1? I.e. the same amount plus half, plus a tenth? So, 55 mi -> 55 + (25 + 2.5) + 5.5 -> 88km And inversely, 0.5 + 0.1, so half plus a tenth: 80km -> 40 + 8 -> 48 miles And for most purposes, just (one and a half = 1.5) and (half = 0.5) is close enough. You can mentally add a little more. So 55mi -> 55 + (25 + 2.5) -> 82 (let's say around 85) 80km -> 40miles (let's say around 45 miles)

JoBrad 7y ago
This is exactly the way I do math in my head.
anongraddebt 7y ago
I often do mental math like this. However, sometimes it is faster to not split up calculations like this. For instance, in the case of converting 55 mi, it's faster (at least, for me) to just divide 54 by 6, move the decimal, and call it a day. I've found that calculations are faster when you know when and when not to split calculations like the way you describe (even though at least 80% of the time it's faster to split them up).
deleted 7y ago
[deleted]

BeetleB 7y ago

I had a Canadian car in the US. My rule was simple. I knew the first few multiples of 16 up to 80. And I knew half of 16 is 8. So to convert 35 miles an hour it's just 48+8. And it's very convenient to know that 50 mi is 80km. Beyond that it's just adding 8s or 16s. For going the other way is just find the nearest multiple of 5 in miles and do a quick approximation. So 70 km. I know that 40 mi is 64 km. Thus 45 mi is 72km. So I'll guess a little lower than 44mi

chibg10 7y ago

1.6 = 8/5 So n miles = 8n / 5 km And m km = 5m / 8 miles Fraction multiplication is actually much easier than decimal multiplication imo. I'm surprised so many people seem to avoid it. Keeping track of where the decimal point should be is a significant cognitive load.