Answer:
t=5
Step-by-step explanation:
y represents height above the ground.
so when it touches the ground height above the ground =0
y=-16t²+80 t
it will hit the ground when y=0
-16t²+80t=0
-16t(t-5)=0
either t=0 or t-5=0
or t=5
t=0 gives initial value.(rejected)
t=5
