Suppose ‘r’ is the rate of interest per annum; Since the interest is compounded half yearly; ∴ R = r/2 and t = 3 years ∴ 3645 = 2560 × (1 + r/200)3 ⇒ (1 + r/200)3 = 729/512 ⇒ 1 + r/200 = 9/8 ⇒ r/200 = 1/8 ⇒ r = 200/8 = 25% ∴ Rate of interest per annum = 25%