Question

A sum of money doubles itself in 4 years at simple interest.How many years will it amount to 8 times of itself

Answer 1

double itself, the money needs to earn 100% interest. So:

100% / 5 years=20%/year

To quadruple, the money needs to make 300%, so:

400%/ 20%/year=20 years

Answer 2

solution is in the above image