> you won't do the long division in your head, so clearly, you'll have to memorise I think this assumption is false; at least for me, doing these types of divi

> you won't do the long division in your head, so clearly, you'll have to memorise I think this assumption is false; at least for me, doing these types of divisions in my head is doable, I think we actually had to be able to do these types of calculations mentally in elementary school. Eg in your examples you say 72/2 = 36, so then 72/4 must be 18. I think the number 72 was chosen specifically for this property, that it divides easily for common values of interest rate (1, 2, 3, 4, 6, 8, 9 all divide cleanly).