A sum of money invested at compound interest doubles itself in 5 years. In how many years will it become eight times itself at the same rate?

Under compound interest the growth factors multiply: after 5 years the sum is 2 times, after 10 years 4 times and after 15 years 8 times. (40 years comes from multiplying 5 by 8, which would only make sense if the growth were proportional to time.)