DRDO Multi tasking staff Model paper 3

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