this has to do with diffrentiation and not kinematics

a boy stands on the edge of a cliff of height 60m. he throws a stone vertically upwards so that its distance h metres above the cliff top is given by h=20t -5(t^2)

a. calculate the max height of the stone above the cliff top

i differentiated h=f(t) and got 20m

b. calculate the time which elapses before the stone hits the beach (it just misses the boy on the way down (lucky him))

i took h=-60 and got t=6

c. calculate the speed with which the stone hits the beach.

here i'm lost. shouldnt speed=f'(t)=20-10t? but i know this is wrong cos the answer is 40ms^-1