Multiple choice

A sum of money lent on simple interest trebles itself in 15 years and 6 months. In how many years will it be doubled?

  1. 6 years and 3 months

  2. 7 years and 9 months

  3. 8 years and 3 months

  4. 9 years and 6 months
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Simple interest formula: SI = P×R×T/100. Amount trebles means 3P = P + SI, so SI = 2P. If this happens in 15.5 years (15 years 6 months), then 2P = P×R×15.5/100, giving R = 200/15.5 = 12.9%. For amount to double: 2P = P + SI, SI = P. So P = P×12.9×T/100, giving T = 100/12.9 = 7.75 years = 7 years 9 months.

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