Overall municipal debt is relatively low and debt burden (the ratio of how much is paid on debt service relative to total budget) averages under 5% according to the most recent U.S. Census data.

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The combination of slow economic growth, a rising debt-to-GDP ratio and a very high ratio of debt to exports make the debt burden unsustainable.

ECONOMIST: Now for round two | The

Portugal started its collapse from a 79% debt-to-GDP ratio in 2010, yet Japan is still easily rolling over a net debt burden of 128% of GDP (latest IMF data).

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