A particle is thrown with a velocity of u m/s.  It passes A and B as shown in figure at time  t1=1s and  t2=3s' The value of u is

If the projectile takes 1 sec to reach A it will take same time 1 sec to come down from B as they are in the same height. So total time of flight is time to reach B and then reach the ground. i.e. 3+1=4 sec.T=2usinθg=4    u=4g2sinθ=40 m/s  θ=300