Question: Suppose that the U.S. net foreign debt is 25% of U.S. GDP and that foreign assets and liabilities alike pay an interest rate of 5% per year. What would be the drain on the US. GDP (as a percentage) from paying interest on the net foreign debt? Do you think this is a large number? What if the net foreign debt were 100% of GNP? When is the debt unsustainable?   Edit