In how many months will Rs. 8,000 yield Rs. 2,648 as compound interest at 20% per annum compounded semi-annually?

Let n be number of half years

Amount = 8000 + 2648 = 10648

From the problem statement

⇒ 10648 = 8000(1 + 0.2/2)n

⇒ 1.331 = (1.1)n

⇒ n = 3

∴ 3 half year means 18 months.