A) 12yrs.
B) 14yrs.
C) 16yrs.
D) 18yrs.
Correct Answer: A
Solution :
Reasoning: If x be the sum it becomes 2x in four years at a certain rate, say r% \[C.I.=A-P=P\] \[2x=x{{\left( 1+\frac{r}{100} \right)}^{3\times 4}}\] or \[2={{\left( 1+\frac{r}{100} \right)}^{4}}\] Cube both sides \[{{R}_{2}}%\] \[{{2}^{3}}={{\left( 1+\frac{r}{100} \right)}^{3\times 4}}\] or \[8={{\left( 1+\frac{r}{100} \right)}^{12}}\] or \[8x=x{{\left( 1+\frac{r}{100} \right)}^{12}}\] Hence, the required number of years = 12 years.
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