Brain Teasers & Mental Math
An investment compounds at 6% per year. Approximately how many times does it multiply over 36 years?
Model answer
About 8× (roughly 2³).
Rule of 72: doubling time ≈ 72 / 6 = 12 years. Over 36 years that is 36 / 12 = 3 doublings, so 2³ = 8×.
Check: 1.06^36 ≈…
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