This is the boss. No calculator — the account pays a clean 10% and the deposit is round hundreds, chosen to fit in your head. One deposit, three years, and you will write the ledger the way a banker once did: each year's interest computed on the CURRENT balance, each new balance feeding the next line. One early slip rolls all the way down the page — and so does one clean line.

Everything in the chapter rides along: simple interest per line, compounding across the lines, the simple-path comparison at the bottom, and the rule of 72 to close the book. 5★ under par here is the chapter crown. Sharpen the pencil.