Brain Teasers & Mental Math
Rule of 72 covers doubling. Roughly how long does money take to TRIPLE and to QUADRUPLE at 9% a year?
Model answer
Triple: ~12.7 years (rule of 114: 114 / 9 ≈ 12.7). Quadruple: 16 years (rule of 144: 144 / 9 = 16).
Why 144 works: quadrupling is just two doublings, and 144 = 2 × 72. Consistency…
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