In a certain sum is doubled in 8 years on S.I in how many years will it be 4 times 

Let sum be $x$ Rs to amount  $2xRs$

So  $S.I=xRs$

$R=\frac{100\times x}{x\times 8}=12.5\%$

Now, for amount  $=4xRs$

$S.I=4x-x=3x$

$T=\frac{100\times 3x}{x\times 12.5}=24years$