Question 54100
You can use a little trigonometry to solve this problem.
Imagine a right triangle in which the perpendicular height of the bridge above the river surface is the height (h) of the triangle. 
The distance (d) from the channel bouy (40 yds) is the base of the right triangle.
The hypotenuse would be the line-of-sight from the bouy to the bridge.
The angle (of elevation) between the base and the hypotenuse was measured at 45 degrees.

Use the fact that the tangent of this angle is the ratio of the side opposite the angle (that's the height of the bridge) over the side adjacent to the angle (that's the base).

{{{Tan(45) = h/40}}} But the tangent of 45 degrees = 1, so:
{{{h/40 = 1}}} and 
{{{h = 40}}}yds. This is the height of the bridge above the river surface.
Since the bungee stretches to twice its measured length, then Jason should use a 20-yard (60-foot) bungee for his jump.