A sum of money becomes thrice of itself in 20 years. Find the simple interest rate per annum?

10%

The correct answer is Option (3) → 10%

Step 1: Let principal = P

• The sum becomes thrice of itself in 20 years

$\text{Amount} = 3P$

• Simple Interest (SI) formula:

$\text{SI} = \frac{P \cdot R \cdot T}{100}$

Here, SI = Total amount − Principal = 3P − P = 2P

$2P = \frac{P \cdot R \cdot 20}{100}$

Step 2: Solve for R

$2P = \frac{20PR}{100}$

$2 = \frac{20R}{100}$

$2 = 0.2 R$

$R = \frac{2}{0.2} = 10\%$