Hello,

By using the formula - N' = N^( T1/T2)

where, N = Number of Times

T = Years

N' = 8 years = 2^3 ( Cube of 2)

N = 2 ( as it doubles)

T1 = 5

T2 = ? ( to find)

Putting the given values in the formula - N' = N^( T1/T2)

2^3 = 2^ (5/T2)

3 = T2/5

15 = T2

So it will take 15 years for the principal amount to becomes 8 times

Hope this was helpful.