Find the least number of complete years in which a sum

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Find the least number of complete years in which a sum of money put out at 20 percent compound interest will be more than doubled ?

1. 2 years
2. 3 years
3. 4 years
4. Data inadequate

Correct Option: C

Here P(1 + 20/10)^t > 2P
⇒ (6/5)^t > 2
∵ (6/5)³ = 1.728, and
(6/5)⁴ = 2.0736
By trial (6/5) x (6/5) x (6/5) x (6/5) > 2
∴ The required time is 4 years.