Answer:
The hunter should aim directly at the perched monkey because the tranquilizer dart will fall away from the line sight at the same rate that the monkey falls from its perch.
Tan theta = 9 / 90 = .1 so theta = 5.71 deg
The time for the monkey to reach the ground is
t = (2 h / g)^1/2 = (18 / 9.8)^1/2 = 1.36 sec
So the horizontal speed of the dart must be at least
Vx = 90 m / 1.36 sec = 66.4 m/s
Vx = V cos theta
V = 66.4 m/s / cos 5.71 = 66.7 m/s
