Question 1007759
dv / dt = a and integrating this we get
v = c + at where c is initial velocity, a is acceleration of gravity and t is time
in our problem c is 0 since the stone is dropped and a = 32 feet/sec^2
now distance covered is the integration of v with respect to time, so
we integrate v = at and get 
distance = (1/2)at^2, therefore
400 = (1/2)* 32 * t^2
t^2 = 800 / 32 
t = 5 seconds, now
v = 32 * 5 = 160 feet per second when the stone hits the ground