A sum borrowed under simple interest doubles itself in 10 years. When will it…

If a sum borrowed under simple interest doubles itself in 10 years, it means that the interest rate is such that the amount of interest earned in 10 years is equal to the principal amount. Let's denote the principal amount as P and the rate of interest as R. Given that the sum doubles itself in 10 years, we can use the formula for simple interest to find R: Simple Interest = P * R * T Since the sum doubles itself, the interest earned in 10 years is equal to the principal amount, so: P * R * 10 = P This simplifies to: R = 1/10 Now, we want to find out when the sum will become fourfold at the same rate of interest. Let's denote the time required for this as T'. Using the same formula for simple interest: Simple Interest = P * R * T' Since we want the sum to become fourfold, the amount of interest earned in T' years should be 3 times the principal amount: P * R * T' = 3P Substituting the value of R we found earlier: P * (1/10) * T' = 3P This simplifies to: T' = 30 years So, it will take 30 years for the sum to become fourfold of itself at the same rate of interest.