What is the united states opportunity cost of producing 1 stereo?

If the US could make 14 stereos/hr or 7 cars/hr, this is a ratio of 2 stereos/1 car, or 1 stereo/0.5 cars. Therefore the US opportunity cost of producing 1 stereo is 0.5 car of half a car.
If China (or Japan? given the question in #2) makes 15 stereos/hr or 5 cars/hr, this is a ratio of 3 stereos/1 car, or 1 stereo/0.33 cars.