From the information given in the question, after time t the inclination of the projectile becomes zero i.e. the projectile is at the highest point. Therefore,
t=2T=gusinθ.
The rang of the projectile R=g2u2sinθcosθ.
Now,
t2R=g2u2sinθcosθ×u2sin2θg2=2gcotθ⇒θ=cot−1(20t2R)