If a certain sum doubles itself after 3 yrs kept at simple interest. In how many yrs will it become 5 times of its value?

Answer 1

(1+r)^3 = 2

2^log_2(5) = 5, so

(1+r)^(3*log_2(5)) = 5

3*log_2(5) = 3 * log5/log2 = 6.97 years

Makes sense, since
3 years = 2x
6 years = 4x
so, 6.97 years = 5x