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A sum of money invested at compound interest amounts in 3 years to Rs 2400 and in 4 years to Rs 2520. The interest rate per annum is 

(a) 5% 

(b) 6% 

(c) 10% 

(d) 12%

1 Answer

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Best answer

(a) 5%

Let the sum of money invested be Rs x and interest rate per annum = r%

Then, x\(\big(1+\frac{r}{100}\big)^3\) = Rs 2400 .....(i)

and x\(\big(1+\frac{r}{100}\big)^4\) = Rs 2520 ........(ii)

Dividing equation (ii) by (i), we get

\(\big(1+\frac{r}{100}\big)\) = \(\frac{2520}{2400}\) \(\Rightarrow\) \(\frac{r}{100}\) = \(\frac{2520-2400}{2400}\) = \(\frac{120}{2400}\)

\(\Rightarrow\) r = \(\frac{1}{20}\)x 100 = 5% p.a

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