How long would it take for a stone to fall 250m traveling at a speed of 10m/s squared?
10 m/s^2 is not a speed. It is an acceleration. You should know that.
When an object falls, it does not have a constant speed (unless air resistance is very high, as with a parachute).
They want you to solve the equation
D = (1/2) g t^2, for time t, using 250 m for D and 10 m/s^2 for g.