Making Change Quickly

Mental Math · Axiom Academy

Nobody behind a register subtracts. They count UP from the price to the cash in your hand — and the change falls out on the way. The register says 13.47 , the customer hands you a 20 , and there are three people in line — how much comes back, and how fast can you say it? Count up to the friendly stops Start at the price and walk up to the cash in your hand, stopping at every friendly number on the way: the next half-dollar, the next whole dollar, then whole dollars up to the bill. Add up what you counted — that is the change, and you never borrowed once. Check it without borrowing a thing Want to be sure before you open the drawer? Take a penny off the twenty first. Subtracting from 19.99 is all 9s underneath, so no column ever borrows — you can even work left to right. Then hand the penny back at the end. Change isn't a number, it's a handful of money. Build it out of the drawer, biggest piece first — the drawer greys out anything that would overshoot. Then look at what came out: the stops you counted up through in Beat 1 are exactly the bills and coins in your hand. Counting up turns a subtraction into a short walk: up to the half-dollar , up to the whole dollar , then whole dollars up to the cash in hand — and what you counted is what you hand back. It works at any register, in any line, with any bill. Away from the register it is the same instinct: when a subtraction looks like it needs borrowing, count up instead , or shift both numbers until it doesn't.

This is the written version of the interactive lesson above. See the full Mental Math course.