Here’s a practical solution based on this:

Take your kilometres e.g 60, divide by 5 -> 12, now times that by 3 -> 36.

Take your miles e.g 80, divide by 5 -> 16, times 8 -> 80 + 48 -> 128

So any conversion reasonable conversion can be done in your head with your 3, 5, and 8 times tables