Let’s suppose the amount invested = Rs.10,000, Time= 5 years and it doubles in the given time, which means the interest rate is 20% per annum.

So, if you’re investing Rs.10,000 at year 1, you will end up receiving Rs.20,000 at the end of year 5.

The formula to calculate above is :

A=P(1+RT)

In your case, the amount you want to receive is Rs. 80,000 (10,000*8); 8 times of the principal.

Hence, using the above formula,

80,000 =10,000{1+(20%)*t}

This means,

T = (1/R)(A/P-1)

= (1/20%)(80000/10000-1)

=35 years

Hence, principally invested at the rate of 20%, would be 8 times in 35 years, if the interest received is @20% per annum.

While, if you receive interest on a compounding basis, the same amount, at the same rate of interest could be received in 12 years on an average, if the compounding is done annually.

Dhwani Mehtaanswered.