A sum of money doubles itself in 4 years compound interest. I will amount to 8 times itself at the same rate of interest in :

Explanation:

Let Principal =P,

Rate % =R %

t=4 years

∴ Amount =2P,

Case1:2P=P(1+R\100)4

2=(1+R\100)^4…………..1

Case2:

Let after t years it will be 8 times

8P=P(1+R\100)t

(2)3=(1+R\100)t……………2

By using eq. 1 and 2

(1+R\100)^12=(1+R\100)t

By comparing both sides,

t= 12 years