Question

Sum of money double itself in 7 years at a simple interest.how many years it will become fourfold

Answer 1

For S.I if it doubles then n×r=100

Sub n=7

7×r=100;

r=100/7

Then for four times

n×r=300

Sub the value of r then

n×100/7=300

Solving the above we get n= 21

Answer 2

It take about 14 years for four folding in which doubling the sum of money takes place in 7 years.

Solution:

Let the principle amount be P, R be the rate of interest and T be the time period

The formula for simple interest is given by: Simple Interest = $\bold{\frac{P T R}{100}}$

$2 P=\frac{P \times 7 \times R}{100}$

On solving we get rate of interest, $R=\frac{200}{7}$

Let in t years the simple interest becomes 4 fold

$\begin{array}{l}{4 P=\frac{p \times R \times T}{100}} \\ \\{T=\frac{400}{R}} \\ \\{T=\frac{400}{\frac{200}{7}}} \\ \\{T=14 \text { years.}}\end{array}$

Therefore it will become fourfold in 14 years.