Making change.

Change isn't subtraction — it's distance. Start at the price, hop up to the next round number, then up to the tendered amount. Adding hops beats borrowing across zeros every time.

Optimal change uses the largest bill or coin that doesn't overshoot. Start big, work down. This is the same algorithm a register prints — train your fingers to follow it.

The cents-portion is its own subproblem. Solve it, get to a clean dollar boundary, then handle the dollar amount. Mixing the two is what causes off-by-one errors.

Customers often hand you "$20.02" on a $7.47 bill so the change is a clean $12.55 instead of $12.53 in mixed coins. Recognize the pattern — the extra cents are intentional, not confusion.

When you hand change back, stack it bills-down then coins-on-top, biggest to smallest. The customer can verify with one glance, and you can too. Speed comes from the muscle memory of always reaching the same way.

They don't do subtraction. They count up from the price to the amount paid. Handed the change out loud, piece by piece, until the running total hits the tendered amount. The subtraction "$20 − $13.47" never happens — instead you walk from $13.47 up to $20 with the bills and coins in your hand, and whatever you hand out is the change.

On each step, use the largest coin or bill that still fits without overshooting. Then stop, check your new running total, and pick the largest piece that fits from there. That's it. The reason it works is that US coin values are stacked so each one is a clean multiple of the last boundary: penny snaps to nickel, nickel to dime, dime to quarter, quarter to dollar.

Smallest to largest: pennies → nickels → dimes → quarters → dollars → larger bills. You start small because the job of the coins is to get you from the odd-cents price to the next clean dollar; once you're there, bills walk you the rest of the way. Going biggest-first is how people overshoot and have to back out — you commit a five, realize you owed $4.30 not $5, and now you're fishing for 70¢ to take back off the counter.

• Starting with the biggest piece. Feels efficient, isn't. You'll overshoot and have to back out. Smallest to largest, every time.
• Defaulting to dimes instead of quarters. If a quarter fits, take the quarter — fewer coins, easier to count back.
• Forgetting the pennies. You can't skip from $13.47 to $13.50 with a nickel. Pennies first, every time the price has odd cents.
• Refusing the customer's extra change. Someone owes $12.47 and hands you a twenty plus seven pennies — take the pennies. Change becomes a single clean bill instead of a fistful of coins.
• Not checking the total before handing it over. Count-up is reliable but a quick glance at the tray total catches a dropped coin.

A working cashier hits 15+ correct in 60 seconds on Medium. A casual target is 10 in 60 seconds. If you're under 5, the rule hasn't clicked yet — drop to Easy, then watch which denomination you reach for first. It should always be pennies (or, if the price is already on a nickel/dime boundary, the smallest coin that does fit).

Power outages and POS failures at stores. Flea markets and garage sales. Tipping the kid who mows the lawn. Cash-only food trucks. Handing your friend $14.75 for their half of the check without pulling out a calculator. Low-stakes, but the kind of skill that quietly pays off the rest of your life.