- All my life I’ve done mental arithmetic the ‘wrong way’: I’ve calculated from left-to-right, instead of right-to-left. So when I do something like 24 + 35 I’ll see the 50 first and then the 9. This even applies when there’s a carry and I’ll do something like 36 + 56 as 80 + 12. I do the same thing for multiplication as well.