Ayoungster throws a rock from a bridge into the river 50 m below. the rock has a speed of 15 m/s when it leaves the youngster’s hand. calculate the velocity of the rock when it strikes the water if it is thrown upward.

Assuming that the stone is thrown vertically... let's say it's a 1 kg stone.It doesn't matter if it's thrown upwards or downwards as (assuming no air friction) it will pass the original throwing point with the same downwards velocity as it had upwards, 3 seconds previously. So it starts with 1/2 m v^2 = 0.5 * 1 * 15^2 = 112.5 J of keThen k.e. gained = gpe lostk.e. gained = m g h = 1 * 10 * 50 = 500 J of Ke gainedso the final (total) ke is 612.5 J which = 1/2 m v^2 = 0.5 v^2 here
so 0.5 v^2 = 612.5so     v^2    = 1225so v = 35 m/s

Ithink the answer is a