Multiple choice

A certain sum doubles in 30 years on simple interest. In how many years will it become 4 times?

  1. 60 years

  2. 90 years

  3. 45 years

  4. 120 years
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Simple interest: Amount = P(1 + RT/100). If it doubles in 30 years, 2P = P(1 + R*30/100), so 1 = 0.3R, R = 10/3 percent. To become 4 times, 4P = P(1 + (10/3)*T/100), so 3 = T/30, T = 90 years.

AI explanation

If a sum doubles in 30 years on simple interest, the interest earned equals the principal in 30 years. To become four times the original amount, the interest earned must equal three times the principal, which takes 3 * 30 = 90 years, making the result 90 years.

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