Rs.625 becomes Rs.1296 in four years at compound interest. The rate of interest per annum is:

20% 25% 15% 12%

20%

In 3 years : Principal : Amount = 625 : 1296 = (5)4 : (6)4 In 1 year : Principal : Amount = \(\sqrt[4]{5^4 }\) : \(\sqrt[4]{6^4 }\) = 5 : 6 Interest = 6 - 5 = 1R Interest rate = \(\frac{1}{5}\) × 100% = 20%