a) 6 years
b) 4 years
c) 8 years
d) 5 years

Answer: b
Explanation:
$$\eqalign{ & {\text{Let principal = P}} \cr & {{Case (I)}} \cr & {\text{Time = 3 years,}} \cr & {\text{Amount = 8P}} \cr & 8{\text{P = P}}{\left( {1 + \frac{{\text{R}}}{{100}}} \right)^3} \cr & {\left( 2 \right)^3} = {\left( {1 + \frac{{\text{R}}}{{100}}} \right)^3} \cr & {\text{Taking cube root of both sides,}} \cr & {\text{2 = }}\left( {1 + \frac{{\text{R}}}{{100}}} \right) \cr & {\text{R = 100 }}\% \cr & {{Case (II)}} \cr & {\text{Let after t years it will be 16 times}} \cr & 16{\text{P = P}}{\left( {1 + \frac{{\text{R}}}{{100}}} \right)^{\text{t}}} \cr & 16 = {\left( 2 \right)^{\text{t}}} \cr & {\left( 2 \right)^4} = {\left( 2 \right)^{\text{t}}} \cr & {\text{t}} = 4 \cr & {\text{Required time}} {\text{(t) = 4 years}} \cr} $$