This question on projectile motion was very popular among the candidates and was fairly well attempted.  However, some candidates did not understand the difference between  projectile from a height and that from a level ground.  Projectile is mostly considered as taking place along a parabolic trajectory and most candidates use the stereotype parabolic equation as against using the linear equations for vertical motion under gravity, few candidates that used the correct equations also substituted wrongly.
The expected answer is:
           Time taken to reach maximum height
           h = ut – ½ gt2
          -50 = 30t – ½ x 10 x t2
            5t2 – 30t – 50  =  0
            t2 – 30 – 10  =  0
            t = 6 ± 
                           2       
            =   7.36 s/7.4 s
                   OR
          V = u – gt
          0 = 30 – 10t
           t =   3s
          Maximum height reached
           V2  =  u2  -  2gx
            0   =  900  -  20x
             x  =  45m.
           Time taken to reach the ground from maximum height
           x  =  ut + ½ gt2
           45 + 50 = 0 + ½  x 10t2
           t  =  19   =  4.4
           Time of flight  =  3 + 4.4   =  7.4s