Velocity of mass at the maximum height = 0 m/s
height = h = 7 m.
the fragments are projected in horizontal direction with velocity = 20 m/s
In the horizontal direction, there is no acceleration. In the vertical direction the acceleration = g = acceleration due to gravity.
since the initial vertical velocity is 0 m/s,
h = 0 t + 1/2 g t²
t = √(2h/g) = time for the fragments to reach the ground.
in this time, the fragments travel 20 * √(2h/g) meters horizontally.