A certain sum becomes 7 times of itself in 10 years at a rate of simple interest. In 20 years it will become how many times of itself at the same rate of simple interest?

Correct option is D

Given:

A certain sum becomes 7 times of itself in 10 years at a rate of simple interest.

Formula Used:

Simple interest = $$\frac{amount \times rate \times time }{100}$$​  

Solution:

Let amount invested =P

Rate = R 

Simple Interest after 10 years  = 7P - P = 6P
6P =$$\frac{10\times P\times R}{100}$$​​
R = 60%
For 20 years,
Simple Interest after 20 years =$$\frac{ 20 \times 60 \times P}{100}$$​​
SI = 12P
Total amount after 20 years = 12P + P = 13P

So, after 20 years amount is 13 times the amount invested.