Brain Teasers & Mental Math
Reverse rule of 72: an investment doubled in 6 years. Roughly what annual compound return did it earn?
Model answer
About 12%.
The rule of 72 works in both directions: rate ≈ 72 / (years to double) = 72 / 6 = 12%.
Exact answer: 2^(1/6) − 1 ≈ 12.25%, so the shortcut is very…
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