A sum of money at compound interest doubles itself in 15 years. It will become four times of itself in-

Question: A sum of money at compound interest doubles itself in 15 years. It will become four times of itself in-Solution: let the sum P 2P = P (1 + r)15⇒ (1 + r)15 = 2let, sum will 4 times in n years4P = P(1 + r)n⇒ 4 = (1 + r)n⇒ 22 = (1 + r)n⇒ ((1 + r)15)2 = (1 + r)n⇒ (1 + r)30 = (1 + r)n∴ n = 30 years