A balloon carrying a basket is descending at a constant velocity of20.0 {\rm m}/{\rm s}. A person in thebasket throws a stone with an initial velocity of 15.0 {\rm m}/{\rm s} horizontally perpendicular to the path of the descending balloon, and 4.00 {\rm s} later this person sees the rock strike the ground. At the instant the rock hits the ground,how far is it from the basket?