From the given data We know amount = p(1 + r/100)n ⇒ 3000(1 + r/100)2 = 6000 ⇒ (1 + r/100)2 = 2 Compound interest for four years = amount – principle = p(1 + r/100)n - principle = 3000(1 + r/100)4 – 3000 = 12000 – 3000 = Rs. 9000 Short Trick Here, amount gets double in 2 years. Hence, in next 2 years amount becomes double of previous amount = in four years amount will become 4 times. Hence, interest = 12000 - 3000 = Rs. 9000