Question

At what rate percent compound interest per annum will Rs 640 amount to Rs 774.40 in 2 years?

Answer 1

Let the rate of interest be R % .
Then, 
\[A = P \left( 1 + \frac{R}{100} \right)^n \]
\[774 . 40 = 640 \left( 1 + \frac{R}{100} \right)^2 \]
\[ \left( 1 + \frac{R}{100} \right)^2 = \frac{774 . 40}{640}\]
\[ \left( 1 + \frac{R}{100} \right)^2 = 1 . 21\]
\[ \left( 1 + \frac{R}{100} \right)^2 = \left( 1 . 1 \right)^2 \]
\[\left( 1 + \frac{R}{100} \right) = 1 . 1\]
\[\frac{R}{100} = 0 . 1\]
R = 10
Thus, the required rate of interest is 10 % per annum.