Projectile motion is when the direction of an object follows the shape of an arc. The only force acting on the object is gravity. Also, the concept is that the motion of the horizontal and vertical directions are independent. Therefore, assuming no air resistance, the time for a rock thrown over the cliff at some angle to reach the ground will be the same time if you just simply free-fall the rock.
Hence, we can solve this using rectilinear motion equations.
y = vt + 1/2*a*t^2, where
y is the vertical distance
v is the initial velocity
a is the acceleration due to gravity equal to -32.2 ft/s^2
t is the time of flight
However, it would be impossible to have 0 height. I think there is some typographical error right there. Let's just assume a height of 1000 ft. Substituting the values,
100 = 160*t + 1/2*-32.2*t^2
t = 9.3 seconds
