This seems to be a economic rather than math question. One possible conclusion to this would be that if china were to allow its currency to strengthen then their goods would not be as cheap here in the United States and thus we would purchase less of their goods given the relative rise in price of their goods,(See a Demand Curve). This also requires an answer to the question are you discussing current account debt, i.e. debt arising from trade imbalances, or is it china's holding of our foreign reserves, i.e. they purchase government bonds issued by the US government. This leads to the further question of which currency the debt is to be repaid in. That is the fundamental factor which determines who benefits and who loses from fluctuations in foreign exchange rates. If you clarify the specifics of what you want answered then I believe I could better answer your question.