Optimisation and Theory of Comparative Advantage
Without more information on producers (such as their production functions) and consumers (their utility functions), we cannot compute the optimal levels of production and trade

The point of this simple numerical example is merely to illustrate the key and surprising insight behind the principle of comparative advantage: Even if the US is better than China at producing everything, both countries (including the US) can still benefit from trade.

The numbers were somewhat arbitrarily chosen (this is a point sharp students are often confused by and the teacher should have mentioned this). They could be tweaked and still successfully illustrate the principle of comparative advantage.