Rule of 72
A mental-math shortcut: divide 72 by an annual growth rate to get the rough number of years for money to double. At 8% a year, that's about nine years — a quick feel for the power (and patience) of compounding.
Where this is taught
Definitions are the trailer. These free levels turn Rule of 72 into something you play — one bite-size lesson, with worked examples, a quiz and XP.