Let initial velocity be u
Total height,H= u2/2g
Total distance the ball travel is u2/g
Total time of flight,T=2u/g
Distance in 1 second
s=u-(g/2)
As given in question
3.6s=2H
3.6 [u-(g/2)]=2×u2/2g
3.6 [u-5]=u2/10
36u-180=u2
When we solve the above equation we get
u=30 or 6
T=2×30/10=6s
Or
T=2×6/10=1.2S
From the given options the correct answer is T=6s