Given :
( Let , take upward direction +ve and downward direction -ve )
Initial speed of stone , u = 4 m/s .
Height , h = 20 m .
To Find : Time taken to reach ground .
Solution :
We know , by equation of motion .
Displacement is given by :
[tex]h=h_o+ut+\dfrac{gt^2}{2}\\\\0=20+4t-2t^2\\\\t^2-2t-10=0[/tex] ( Here , g = acceleration due to gravity = [tex]-9.8\ m/s^2[/tex] .)
Solving above equation , we get :
t = 4.32 s .
Hence , this is the required solution .
