Compound Interest

Learner likely understands interest as a flat fee or percentage of an original amount, but has not yet built intuition for why repeatedly reinvesting earned interest produces exponential — not linear — growth over time. Principal & Rate: Ask: if you borrow $500 at 8% annual interest, what two numbers go into the compound interest formula and what do they represent? Compounding Period: Ask: why does compounding monthly produce more growth than compounding annually at the same stated rate? What changes in the formula?

The gap most people have on compound interest is the part that actually changes outcomes: Learner likely understands interest as a flat fee or percentage of an original amount, but has not yet built intuition for why repeatedly reinvesting earned interest produces exponential — not linear — growth over time. Once that lands, the supporting ideas — effective annual rate — start paying off in everyday decisions.

Many people first hear "Compound" and think of combining or layering things together (like a compound mixture). In compound interest, 'compound' means interest is added on top of prior interest, so each period layers new growth onto an already-grown base — the layering intuition maps perfectly. Many people first hear "Interest" and think of a fee the bank charges you for borrowing money. Compound interest applies equally to savings and loans: when you save, the bank pays you interest on your growing balance; when you borrow, you owe interest on a growing balance — the math is identical, the direction just flips.

LearnBench teaches compound interest in 6 adaptive cards organized around 4 core ideas. A few quick checks find what you already know, then the lesson skips it — so you only see the parts you're actually missing, framed with concrete analogies.