Here is the entire lesson: money at $R$ percent a year doubles in about $\dfrac{72}{R}$ years. Two letters, and one of them has a twist: $t$ is the wait in years, and $R$ is the rate as its percent number — 6 for 6%, not the decimal 0.06 that every other formula in this chapter wants. That is the rule of 72 — read aloud: t is about 72 over R — and it runs on mental math alone, which is why bankers love it and calculators fear it. At 6%, about $72 \div 6 = 12$ years. At 9%, about 8. At 12%, about 6.

Why 72? It is compounding's own number: close to the true mathematics, and divisible by nearly everything — 2, 3, 4, 6, 8, 9 and 12 all go in evenly. Notice the $\approx$ in $t \approx \dfrac{72}{R}$: the rule is an honest estimate, not an exact law. On an exam it is your five-second sanity check. Here, it IS the question.