Trisha - sketch a picture of the Earth with the observer 30 ft above it. Use the radius of the Earth (4000 miles) to find your answer.
Here is sketch of the problem:
The distance BD and BA are both radii of the Earth or 4000 miles. The distance DC is 30 feet or
The Observer is at point C, so find the distance CA which is the tangent line to the Earth. The line BA will be perpendicular to CA forming a right angle since all radii to a tangent line are 90 degrees.
Using the Pythagorean Theorem we can find the distance CA which the Observer can see in one direction:
Now, the Observer can look in any direction over the water and see 6.742 miles. The area that the Observer can see forms a circle with radius 6.742 miles.
Total Area of Ocean =
hope that helped