Multiple choice

An amount becomes 3 times in 4 years when invested in a scheme under simple interest. In how many years will it become 21 times?

  1. 20

  2. 36

  3. 40

  4. 30
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Simple interest formula: A = P(1 + rt). If amount becomes 3 times, 3P = P(1 + r*4), so 2 = 4r, r = 0.5 (50% per year). To become 21 times, 21P = P(1 + 0.5*t), so 20 = 0.5*t, t = 40 years.

AI explanation

If a sum becomes 3 times in 4 years, it earns 2 times the principal as interest. This means the interest per year is 2P / 4, or 0.5P. To become 21 times, the interest must be 20 times the principal. The required time is 20P divided by 0.5P, which equals 40 years.

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