What is the most interesting mathematics question you've answered on here?

A cargo ship and a Coast Guard ship leave Morehead, NC at the same time, 10:45 a.m. The cargo ship sails due east at a constant speed of 18 knots and the Coast Guard ship sails due south at a constant speed of 38 knots. At 3:30 p.m., the Coast Guard ship receives a distress call from the cargo ship, which is dead in the water. The Coast Guard ship dispatches a helicopter, which flies along the path of the cargo ship's radio signal. At the same time, the Coast Guard ship turns to the same heading and sails toward the stricken cargo ship.

The helicopter burns 90 gals. of fuel per hour, its fuel tank is loaded with 450 gals. of fuel and has a maximum ground speed of 130 knots. How long can the helicopter stay on station before having to head back to the Coast Guard ship without crashing into the sea?

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Countless Calculus questions or Physics problems.