The potential energy of a projectile at it's maximum height is equal to it's kinetic energy there if the velocity of projection is 20m/s it's time of flight
m g h = 1/2 m v^2 ... h = v^2 / (2 g) = 200 / g
1/2 g t^2 = 200 / g ... t = 20 / g
t is half of the time of flight