At what simple interest rate per annum will a sum of money become four times of itself in 20 years?

Correct option is D

Given:

Amount becomes 4 times the principal in 20 years.
Time = 20 years

Formula Used:
Simple Interest (SI) = Amount – Principal

SI $$= \frac{P \times R \times T}{100}$$​

Solution:

Let Principal = P, then Amount = 4P

SI = 4P - P = 3P

​$$\frac{P \times R \times 20}{100} = 3P$$​​

​$$\frac{20R}{100} = 3$$​​

R $$= \frac{3 \times 100}{20} =$$​ 15 % per annum

Alternate Method:

R %= $$\frac{(n-1)}T \times 100\%$$ 

R % = $$\frac{4-1}{20}\times 100\%= \frac3{20}\times 100\%= 15\%$$​​