The Fibonacci sequence can help you quickly convert between miles and kilometers
The Fibonacci sequence is a series of numbers where every new number is the sum of the two previous ones in the series.
1, 1, 2, 3, 5, 8, 13, 21, etc.
The next number would be 13 + 21 = 34.
Here’s the thing: 5 mi = 8 km. 8 mi = 13 km. 13 mi = 21 km, and so on.
Edit: You can also do this with multiples of these numbers (e.g. 5*10 = 8*10, 50 mi = 80 km). If you’ve got an odd number that doesn’t fit in the sequence, you can also just round to the nearest Fibonacci number and compensate for this in the answer. E.g. 70 mi ≈ 80 mi. 80 mi = 130 km. Subtract a small value like 15 km to compensate for the rounding, and the end result is 115 km.
This works because the Fibonacci sequence increases following the golden ratio (1:1.618). The ratio between miles and km is 1:1.609, or very, very close to the golden ratio. Hence, the Fibonacci sequence provides very good approximations when converting between km and miles.