Let the sum (P) be x. It becomes double means 2x in 8 (T) years.
It means Interest (I) = 2x - x = x. we have to fine the rate of interest
(R) from here.
We know, I = PRT/100
Again, Sum (P) = x, sum + interest = 4x. Interest (I) = 4x - x = 3x
Rate of interest (R) = 100/8 and we have to find no. of years (T)
We know I = PRT/100