Question

At what rate of annual simple interest, a certain sum will amount to four times in 15yr

Answer 1

use a random number to use for the sum Ex. 300 is going to be the sum we are using and then Im going to use i for the interest rate so it doesn't get confusing with the variables
300=sum
i=interest rate
300xi^15=300x4
300xi^15=1200
i^15= 4
15square root of 4 = 1.09682498

Answer 2

first, simple interest, not compound
second, no random amount needs to be devised. We just want

(1+15r) = 4
r = 0.2 = 20%

sounds right, since at 20%, the original amount is added every 5 years. So, after 15 years, that has happened 3 times, making the balance 4 times the original amount.