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Fun fact: Phi (~1.618) is really close to the ratio between miles and kilometers (~1.609), which means you can use adjacent fibonacci numbers to quickly mentally convert back and forth between them.

for instance: 21 kilometers is roughly 13 miles (it's actually 13.05)

@00dani Cursed indeed. If only the US americans could decide to arrive in the present time like all of the other nations and use the sensible SI units.
@jacethechicken

@jacethechicken That's kinda cool, although you need the quantity you want to convert to be close enough to a Fibonacci number 😄

@otini You can use simple doublings or halvings to get close, or you can use more than one. So 42km is 34km + 8km, which is therefore 21+5 miles, or 26 miles.

The correct ratio is almost exactly ln(5), but that doesn't help many people.

@jacethechicken

@otini The other option for 42km is that it's double 21km, which is 13 miles, so the answer is double 13 miles, which is 26 miles.

Nice that the answers agree!
@jacethechicken

@jacethechicken When I see something like this, I'm always reminded that things I've known for decades are new to folk, and I really should make a list and start to share them more regularly.

For example ... consider where cos(x) and tan(x) cross. At what angle do they cross, and what are their gradients?

@jacethechicken I can never remember whether phi is the 1.618-something number or the 0.618-something number but for estimation it turns out it doesn't matter -- they're inverses of each other. :-O

@jacethechicken related: you can roughly convert Fahrenheit to centigrade by halving then subtracting 30

(Here inaccuracies are bigger and have more significant effects!)

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