Ya got me. Ok, how about this. You go to the hardware store to buy a hammer, and it comes to $12.37. You hand the clerk a $20. The clerk begins counting out your change, out loud, right on the counter in front of you.
12.37 + 3 cents is 12.40.
12.40 + 10 cents is 12.50.
12.50 + 50 cents (two quarters) is 13.
13 + 2 is 15.
15 + 5 is 20.
I'm 50 years old. Before cell phones, before calculators, before wallets full of credit cards, before cash registers calculated change for the clerk, this is how change was made, right on the counter in front of the customer, who was doing the math right along with the clerk to make sure they got the correct change. This isn't new math, this is the way math was done for eons.