A sum of money doubles itself in 8 years at compound interest. In how many years will it become 4 times itself?

💡 Solution & Explanation

With compound interest the sum keeps doubling every 8 years: 2 times after 8 years, 4 times after 16, 8 times after 24, and so on. 4 = 2^2, so it takes 2 doublings: 2 × 8 = 16 years. Multiplying 8 by 4 (32 years) would be the simple-interest way of thinking, which does not apply here.