A sum of money doubles itself in 10 years at compound interest. In how many years will it become 16 times itself?

💡 Solution & Explanation

With compound interest the sum keeps doubling every 10 years: 2 times after 10 years, 4 times after 20, 8 times after 30, and so on. 16 = 2^4, so it takes 4 doublings: 4 × 10 = 40 years. Multiplying 10 by 16 (160 years) would be the simple-interest way of thinking, which does not apply here.