Correct Answer:
B. 10 years
This question involves simple interest calculations. Let the principal amount be P and the annual simple interest rate be R. The formula for simple interest is I = PRT/100.
- Step 1: Find the interest rate (R). If the investment triples in 20 years, the interest earned (I) is 2P (since P becomes 3P). So, 2P = P * R * 20 / 100. This simplifies to 2 = R * 20 / 100, which means R = 10%.
- Step 2: Find the time (T) to double the investment. To double the investment, the interest earned (I) needs to be P (since P becomes 2P). Using the rate R=10%, we have P = P * 10 * T / 100. This simplifies to 1 = 10 * T / 100, which means T = 10 years.
Therefore, the correct answer is 10 years. The other options are incorrect calculations based on the principles of simple interest.