I'm trying to understand the mark scheme for the following question:
A small stone is projected from a point O at the top of a vertical cliff OA. The point O is 52.5 m above the sea. The stone rises to a maximum height of 10 m above the level of O before hitting the sea at the point B, where AB = 50 m. The stone is modelled as a particle moving freely under gravity, as shown in the diagram below:
It can be shown that the vertical component of the velocity of projection of the stone is 14 m/s.
Find the time after projection when the stone is moving parallel to OB.
The mark scheme says: 
How do they know that Uh is 10? What even is it? Where did it come from?
Thanks :)
The time it takes the stone to move 50 meters is equal to the time it would take an object to fall 10 meters (1.43 sec.) plus the time it would take an object to fall 62.5 meters (3.57 sec.) So it moved 50 meters horizontally in 5 seconds.