From a mountain 1780 ft. high, the angle of depression of a point on the nearer shore of a river is 48 degrees 40 minutes and of a point directly across on the opposite side is 22 degrees 20 minutes. What is the width of the river between the two points?

Answer 1

If you draw a diagram, you will see that you cannot compute the width of the river without knowing how far the river is from the mountain.

Answer 2

If we assume that the river is at the same elevation of the base of the mountain, then
Width of the river
=distance of far shore - distance of near shore
=1780/tan(θ2)-1780/tan(θ1)