Excerpt:Â How much change does a grocery customer get if he hands over a $20 bill to pay for $7.93 in groceries? How many of you instantly said $12.07?
Good. But what if your customer said, "Oh, and here are three pennies?"
How many of you said you'd hand him $12.10?
Excellent! This simple mental arithmetic would qualify you to work as a cashier any time from the minting of the first coins â€“ sometime around 600 B.C. â€“ until the advent of electronic calculators.