I learned to at least count change in a cash register. 45 cents is 54 back. 35 is 65 back. It's rote memorization, like multiplication. 100, minus the total change, and you have made a dollar. At $7.01 you give them 2 dollars and 99 cents from a ten. Anything added to 7 decreases the dollar amount one, then count back from 100. If someone has an 18.43 bill, and wants a five back, it's 19 dollars. So, give me 24 and I'll give you $5.57 back. That's math I do in my head. All day. I check my register, the register checks me.