Initial velocity of ball u = 30 "\\frac{m}{sec}"
gravitational acceleration g = 1.60 "\\frac{m}{sec^{2}}"
using equation motion
The maximum height reached by ball
"v^{2}-u^{2} = -2gh"
"0^{2}-30^{2} = - 2\\times 1.60 \\times h" ( Final velocity v = 0, Negative sign is because of deceleration or oppose to gravity )
"h =" 281.25 m
The time taken to reach that height.
"v = u - gt"
"0 = 30 -1.60\\times t"
"t =" 18.75 sec
Its velocity 20s after it is thrown.
"v = u - gt"
"v = 30 -1.60 \\times 20"
"v = -2 \\frac{m}{sec}"
When the ball’s height is 50m
"v^{2}-u^{2} = -2gh"
"v^{2}-30^{2} = -2\\times 1.60 \\times 50"
"v = 27.20 \\frac{m}{sec}"
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