To solve this problem, we must imagine that Jim’s initial position, the position of the rock, and Jim’s final position all connects to form a triangle. Now we can imagine that the triangle is a right triangle with the 90° angle on the initial position.
The angle of 30° is directly opposite to the length of his total stride while the width of the river is the side adjacent to the angle. Therefore can use the tan function to solve for the width of the river:
tan θ = opposite side / adjacent side
tan 30 = total stride distance / width of river
where total stride distance = 65 * 0.8 = 52 m
width of river = 52 m / tan 30
width of river = 90.07 m