A sum of money invested at compound interest amounts to ₹ 800 in 3 years and to ₹ 840 in 4 years

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6.Interest

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A sum of money invested at compound interest amounts to ₹ $800$ in $3$ years and to ₹ $840$ in 4 years. The rate of interest (In $\%$) per annum is

A

$2.5$

B

$4$

C

$5$

D

$6.67$

Solution

Let $R$ be the rate of interest. We have,

$A 1=P\left(1+\frac{R}{100}\right)^{3}=800$

$P\left(1+\frac{R}{100}\right)^{3}=800$

Similarly $P\left(1+\frac{R}{100}\right)^{4}=840$

$\frac{(2)}{(1)} \Rightarrow$

$\left(1+\frac{R}{100}\right)=\frac{840}{800}=1.05$

$\therefore$

$R=5 \%$

Standard 13

Quantitative Aptitude

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