Question

A sum of money doubles in 5 years at simple interest. In how many years will it become 4 times?

• A. 10 years
• B. 12 years
• C. 15 years
• D. 20 years

Hint:

For simple interest, time is proportional to the interest earned.

Answer 1

The Correct Option is C

- Step 1: Let principal = $P$, rate = $R%$. Simple interest for 5 years makes amount = $2P$.
- Step 2: Interest = $P$, so $P = \frac{P \times R \times 5}{100} \implies R = 20%$.
- Step 3: For amount to be $4P$, interest = $3P$. Time $T$ satisfies: $3P = \frac{P \times 20 \times T}{100} \implies 3 = \frac{20T}{100} \implies T = 15$.
- Step 4: Verify: In 15 years, interest = $\frac{P \times 20 \times 15}{100} = 3P$, amount = $P + 3P = 4P$.
- Step 5: Check options: Option (3) is 15 years, correct.