Question

A sum of money placed at C. I. doubles itself in 5 years. It will amount to eight times itself in ............

• A. 15 years
• B. 20 years
• C. 12 years
• D. 10 years

Answer 1

Answer: (A)

15 years

Let the amount Rs 100 and rate of interest r
By hypothesis
$200=100(1+\frac{r}{100}{)}^5$
$\Rightarrow 2=(1+\frac{r}{100}{)}^5$
$\Rightarrow (1+\frac{r}{100})=2^{\frac{1}{5}}$
Lets amount become 8 time in n years then
$800=100(1+\frac{r}{100}{)}^n$
$\Rightarrow 8=(1+\frac{r}{100}{)}^n$=$(2^{\frac{1}{5}}{)}^{{}^n}$
$\Rightarrow (2{)}^3$=$(2{)}^{\frac{n}{5}}$
$\therefore \frac{n}{5}=3$
OR n=15 years