A car moving with a speed of 50 kmph can be stopped by brakes after atleast 6m. If the same car is moving with 100 kmph, the minimum stopping distance is

Using work energy theorem, 

the work done by friction will be the change in kinetic energy

$\mathrm{f}.\mathrm{s}=∆\mathrm{K}.\mathrm{E}$

as the frictional force is same in both the cases

$\Rightarrow \frac{s_1}{s_2}=\frac{K.E_1}{K.E_2}=\frac{{v_1}^2}{{v_2}^2}$

$\Rightarrow {\mathrm{s}}_2=\frac{(100{)}^2}{(50{)}^2}6=4\times 6=24\mathrm{m}$