A sum of money placed at compound interest doubles itself in 4 years. In how many years will it amount to 8 times?  Choose the correct option:
A) 10 years
B) 8 years
C) 6 years
D) 12 years
E) 13 years
Explanation:   Let the sum be Rs.1 which becomes Rs.2 after 4 years   ⇒ 2 = 1(1 + R/100 )4 --- (equation 1)   Let the sum of Rs.1 becomes Rs.8 after n years   ⇒ 8 = 1(1 + R/100)n --- (equation 2)   ⇒ (2)3 = 1(1+R/100)n   ⇒ [1(1+R/100)4]3 = 1(1+R/100)n (âˆµ replaced 2 with the value in equation 1) (1 + R/100)12 = (1+R/100)n   ⇒ n = 12   i.e., the sum amounts to 8 times in 12 years   Answer: Option 12 years
<result>D</result>
A sum of money placed at compound interest doubles itself in 4 years. In how many years will it amount to 8 times?