The total time of flight is given as
T=2usinθgT = \frac{2u \sin\theta}{g}T=g2usinθ
Hence, time to maximum height is half of the total time of flight,
t = usinθg\frac{u \sin\theta}{g}gusinθ
= 300sin3010\frac{300 \sin 30}{10}10300sin30 = 15s
