The following table gives the U S national debt for selected years from 1900 to 2013 U S Debt billions 3233 5225 5674 7933 11 957 13 529 15 476 U S Debt billions 1 1 24 16 43 259 273 314 Year 1900 1910 1920 1930 1940 1945 1955 1965 1975 533 1985 1823 a Using a function of the form y a b x with x 0 in 1900 and y equal to the national debt in billions model the data Round your coefficients to four decimal pla Year 1990 1996 2000 2005 2009 2010 2011 2012 2013 16 067 16 856 b Use the model to predict the debt in 2021 Round your answer to the nearest billion billion c Predict the year in which the debt will be 99 trillion 99 000 billion