A sum of money placed at compound interest doubles itself in 5 years in how many years it would to 8 times of itself at the same rate of interest?

A. 7 years
B. 10 years
C. 15 years
D. 20 years
Explanation:
Let principal be Rs x and the rate is R % p.a.
Then X × (1 + R/100)5 = 2x
=> (1 + R/100)5 = 2
Let x × (1 + R/100)t = 8x
=> (1 + R/100)t = 8 = 23
={(1 + R/100)5 }3
=> (1 + R/100)t = (1 + R/100)15
=> T = 15 Years