A body, starting from rest, attains a velocity of $20 m{s}^{-1}$.The time taken by the body in the displacement of 50 m is                                   

$$\begin{gathered} s=ut+1/2a{t}^{2 } v^2-u^2=2as \\ 50=\frac{1}{2}4{\left(t\right)}^2 {20}^2=2\left(a\right)\left(50\right) \\ t=5sec 20\times 20=100a \\ a=4m/s^2 \end{gathered}$$