Adam drops a ball from rest from the top floor of a building at the same time Bob throws a ball horizontally from the same location. Which ball hits the ground first

Both hit ground at the same time.

Explanation:

As Bob throws the ball horizontally, this means that in the vertical direction, the initial velocity is 0.Now, as Adam drops a ball from rest, this means that its initial velocity is 0 also.So, for the vertical movement (which is independent from the horizontal one due to they are perpendicular), for both balls, the equation for displacement is the same, as both balls start from rest and only under the influence of gravity which accelerates them alike.We can conclude that being the displacement the same, the initial velocity the same and the acceleration also the same, both balls hit ground at the same time, at different points, due to the horizontal movement of one of them.