A sum of money doubles itself in 4 years at compound interest. In how many years will it become 16 times itself?

💡 Solution & Explanation

With compound interest the sum keeps doubling every 4 years: 2 times after 4 years, 4 times after 8, 8 times after 12, and so on. 16 = 2^4, so it takes 4 doublings: 4 × 4 = 16 years. Multiplying 4 by 16 (64 years) would be the simple-interest way of thinking, which does not apply here.