A body is allowed to fall from a height of  100m. After 2s if acceleration due to gravity vanishes the total time of its fall is $\left(g=10m/s^2\right)$ in sec

Distance travelled in 2s is $h=\frac{1}{2}g{t}^2$ 
                                  $=\frac{1}{2}\times 10\times 2^2=20m$
Velocity acquired after 2s is $v=gt$ 
                                  $=10\times 2=20m/s$
As there is no acceleration, that body could travel the remaining distance $\left(100-20=80m\right)$ with uniform velocity of 20m/s.
$\therefore$ The time taken for the remaining journey is $t=\frac{80}{20}=4s$.
$\therefore$ Total time of motion is $2+4=6s$