Multiple choice general knowledge math & puzzles In how many years a sum of money DOUBLES itself at 4% per annum 8 16 25 20 Reveal answer Fill a bubble to check yourself C Correct answer Explanation For money to double at 4% compound interest: 2 = (1.04)^n. Testing n=25: 1.04^25 ≈ 2.66 > 2, so 25 years works. This uses the compound interest formula A = P(1+r)^n.