You drop two objects of different masses simultaneously from the top of a tower. Show that, if you assume the air resistance to have the same constant value for each object, the one with the larger mass will strike the ground first. How good is this assumption?

In the absence of air resistance, both the objects fall with same acceleration and reach the ground at the same time. However in the presence of air resistance, the falling object experiences an upward acceleration. The net acceleration of the object is the difference in the magnitude of its downward and upward acceleration and is the key to account for the time of flight of objects.
Because of larger inertia, it will be harder for the force from air resistance, to accelerate the more massive object relative to the less massive one. Therefore the upward acceleration experienced by the heavier object will be less than the upward acceleration experienced by the lighter one.
Thus, the net downward acceleration of the heavier object would be greater than the net downward acceleration of the lighter one. This accounts for the fact that the more massive object will reach the ground first.