Very strangely worded question, with two obvious mistakes / meaningless terms ("forming a 20 angle of depression" / "to the nearest thousandth"?).
But this is a trigonometry question, not inverse:
- Draw a straight horizontal line (the ground)
- Mark the plane somewhere above it. Draw a straight vertical line between the ground and the plane.
- Mark the island on the ground, somewhere ahead of the plane. Now join the plane and island with a diagonal line. This is the length you need to solve.
- You have a right-angled triangle now, with an angle at the island vertex of 20 degrees (assuming the question means that the line of sight from the plane to the island forms a 20 degree angle of depression; the way they have written it is meaningless nonsense).
So, you can solve the length using the property sin(theta) = o/h. Theta is 20 degrees, opposite side to the angle is the height of the plane, and the hypotentuse is what you're calculating.
You should get h = 20,500 / sin(20 deg)
(I wonder if they mean round to the nearest "thousand feet"... in which case it's 58,000 feet).
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