4 ms·

As a programmer, I am much more facile with powers of 2, than Fibonacci. My way is to double the number 4 times (multiplying by 16) and divide by 10. So for ex

As a programmer, I am much more facile with powers of 2, than Fibonacci. My way is to double the number 4 times (multiplying by 16) and divide by 10. So for example 55 mph, double 4 times: 110, 220, 440, 880 divide by 10: 88 So 55 mph is approximately 88 km/h

biggio 7y ago
This is the method I'm using :D
mabbo 7y ago
Yes! I'm a Canadian whose brain works in km but I spent a year and a bit working a job that had me flying to (and subsequently driving around) places across the USA. All the time I'd be finding myself going "Okay, 30 mph... what the heck is that?". Double four times, and divide by ten as soon as possible (ie: the first time you see a zero on the end, drop it). So 30 mph => 3 (drop the zero) => 6 => 12 => 24 => 48 km/h The nice thing about this method is that you know that the kph is going to be more than mph, but less than double it, so you don't have to count the doublings very well- when you're in the right range, you're at the right number. Example: 135 miles to destination is how many km? Okay, double to 270; drop the zero to 27; double to 54; double to 108;... we're still less than the mph, so we must need to double again to get 216 km. Now we're between 135 and 2*135, so we must be at the answer. 216 km. (actual answer is 217.3).
- crdrost 7y ago
  You can also add a half and a tenth. 135, half is 67, a tenth is 13, add those two together to get 80, 135+80=215.
hangonhn 7y ago
This is so brilliant! As a runner (most distances in running are measures in meters) and a programmer, this is just perfect! Thanks for sharing!
vecter 7y ago
Good trick. A little easier: divide by 10 first and then double four times.
palimpsests 7y ago
so in other words, you multiply by 1.6. How is having facility with powers of 2 helping you here other than knowing that 2^4 = 16? I mean, I get what you're saying in that doubling a value several times is relatively easy mental math, and at the same time, this whole thread strikes me as overcomplicating something that doesn't need this much complexity and abstraction.

biggio 7y ago

This is the method I'm using :D

mabbo 7y ago

Yes! I'm a Canadian whose brain works in km but I spent a year and a bit working a job that had me flying to (and subsequently driving around) places across the USA. All the time I'd be finding myself going "Okay, 30 mph... what the heck is that?". Double four times, and divide by ten as soon as possible (ie: the first time you see a zero on the end, drop it). So 30 mph => 3 (drop the zero) => 6 => 12 => 24 => 48 km/h The nice thing about this method is that you know that the kph is going to be more than mph, but less than double it, so you don't have to count the doublings very well- when you're in the right range, you're at the right number. Example: 135 miles to destination is how many km? Okay, double to 270; drop the zero to 27; double to 54; double to 108;... we're still less than the mph, so we must need to double again to get 216 km. Now we're between 135 and 2*135, so we must be at the answer. 216 km. (actual answer is 217.3).

crdrost 7y ago
You can also add a half and a tenth. 135, half is 67, a tenth is 13, add those two together to get 80, 135+80=215.

hangonhn 7y ago

This is so brilliant! As a runner (most distances in running are measures in meters) and a programmer, this is just perfect! Thanks for sharing!

vecter 7y ago

Good trick. A little easier: divide by 10 first and then double four times.

palimpsests 7y ago

so in other words, you multiply by 1.6. How is having facility with powers of 2 helping you here other than knowing that 2^4 = 16? I mean, I get what you're saying in that doubling a value several times is relatively easy mental math, and at the same time, this whole thread strikes me as overcomplicating something that doesn't need this much complexity and abstraction.