Brain Teasers & Mental Math
Rule of 72: roughly how long does it take money to double at 8% annual compounding? At 12%?
Model answer
At 8%: about 72 / 8 = 9 years. At 12%: about 72 / 12 = 6 years.
Rule of 72: doubling time (years) ≈ 72 / (interest rate in %). It works because ln(2) ≈ 0.693, and 72 is…
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