Question

which the following is correct )))??? (see image questions full ))

Answer 1

hi dear your answer is 20s
hope it is helpful to you..

Answer 2

time of flight is due to vertical velocity,so 
s= ut+ 1/2 at^2 
initial vertical velocity is 0, so 
s= 1/2 at^2 
or 490= 1/2*9.8(acc. due to gravity)* t^2 
or 100= t^2 
or t =10sec