Re: We use base 10 for a reason
I did O-levels (pre GCSE!) in the good old pounds, shillings and pence days. I had to learn how to add, subtract, multiply and divide sums in pounds, shillings and pence. However, the only sensible way to do compound interest calculations was to convert to pounds and decimals of a pound, do the calculation, then convert back! However, there were a lot of useful hacks for less demanding arithmetic, some of which I can still remember - the dozen rule, for example (things were often sold in multiples of 12 in those days, so knowing that the price of 12 items in shillings = price of one item in pennies was very useful), the score rule (same in shillings and pounds), and several others I've now forgotten!