Multiple choice

Dhansukhlal deposited money in a cooperative bank which promised to quadruple his amount in 20 years under simple interest. Dhansukhlal decided to withdraw his money the moment it tripled. After how much time should he withdraw his money from the bank? [Interest is calculated per annum]

  1. 12 years 6 months

  2. 13 years 4 months

  3. 13 years

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Simple interest: Amount = P + P*R*T/100. Quadrupling means 4P = P + P*R*20/100, so 3 = R/5, R = 15%. Tripling means 3P = P + P*15*T/100, so 2 = 15T/100, T = 200/15 = 13.33 years = 13 years 4 months.

AI explanation

If the money quadruples in 20 years under simple interest, the interest earned is three times the principal, meaning 3P = (P * R * 20) / 100 and the rate of interest is 15% per annum. For the money to triple, the interest earned must be twice the principal, so 2P = (P * 15 * T) / 100. Solving for T gives T = 200 / 15, which is 13.33 years or 13 years and 4 months.

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