The U.S. public debt was $909 billion in 1980, an amount equal to 33% of Americas gross domestic product (GDP); by 1990, that number had more than tripled to $3.2 trillion—or 56% of GDP. In 2001 the national debt was $5.7 trillion; however, the debt-to-GDP ratio remained at 1990 levels. Debt levels rose quickly in the following decade, and on January 28, 2010, the U.S. debt ceiling was raised to $14.3 trillion. Based on the 2010 United States federal budget, total national debt will grow to nearly 100% of GDP, versus a level of approximately 80% in early 2009. The White House estimates that the governments tab for servicing the debt will exceed $700 billion a year in 2019, up from $202 billion in 2009.
Question: How many years did it take for the national debt to increase from $909 billion in 1980 to $5.7 trillion?
Answer: 21