If a principal amount invested at compound interest doubles in 5 years, how long will it take to become eight times the original amount at the same interest rate?

Explanation

Given that the amount doubles in 5 years, we have (1 + r/100)^5 = 2. To find the time n when the amount becomes eight times, set (1 + r/100)^n = 8. Since 8 equals 2 raised to the power 3, we get (1 + r/100)^n = (1 + r/100)^(5×3). Therefore, n = 15 years.