At 20% per annum rate, an amount is doubled in approximately in ______ years at compound interest.

Question

Question

This question was previously asked in

RPF Constable 2024 Official Paper (Held On 05 Mar, 2025 Shift 3)

Answer 1

Given:

Rate (r) = 20% per annum

Amount becomes double ⇒ A = 2P

Formula used:

A = P(1 + r/100)^t

Calculation:

⇒ 2P = P(1 + 20/100)^t

⇒ 2 = (1.2)^t

Now, take logarithm both sides:

⇒ log(2) = t × log(1.2)

⇒ t = log(2) ÷ log(1.2)

⇒ t = 0.3010 ÷ 0.0792

⇒ t ≈ 3.8 years

∴ The amount is doubled in approximately 4 years.