A sum of money lent at simple interest doubles itself in 8 years. In how many years will it become five times itself at the same rate?

Doubling means the interest earned equals the principal in 8 years, so the rate is 100/8 = 12.5% per annum. To become five times, the interest must be 4 times the principal, needing $4\times 8=32$ years. (40 years comes from multiplying 8 by 5 and forgetting that the principal is already there.)