Explanation:
The vertical component the velocity of the projectile is 15 m/s x sin 30 = 7.5 m/s.
The body is accelarating downwards at 10 m/s^2.
This means that every second its upward velocity reduces by 10 m/s.
So if the body is travelling upwards at 7.5 m/s then how long does it take for the velocity to become 0?
(7.5 m/s) / (10 m/s^2) = 0.75 s
