This question was a popular one among the candidates and was correctly solved by many candidates. However few candidates exhibited poor computational skills in the handling of square and square roots of angles in the formula used. Some even used wrong formula.
The expected answer is:
The projectile takes 2.1 seconds to reach the maximum height H.
H = (U sinθ) t - ½ qt2
= (42 sin 30) (2.1) - ½ x 10 x (2.1)2
= 44.1 - 22.05
H = U2 sin2θ
= 422 x ( sin 30)2
2 x 10