Answer: 565 feet
This value is approximate.
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Step-by-step explanation:
Imagine you are at the top of the lighthouse, and look straight horizontally outward. Then imagine looking down 23 degrees to spot the boat. This forms the angle of depression.
The adjacent angle complementary to this is 90-23 = 67 degrees. This is the interior angle of the triangle needed, and this angle is at the top of the triangle.
tan(angle) = opposite/adjacent
tan(67) = x/240
x = 240*tan(67)
x = 565.4045677977 approximately
x = 565 feet
The diagram is below.