Assume the United States's national debt can be estimated with the model y = 0.051517(1.1306727)x, where x represents the number of years since 1900 and y represents the debt in billions of dollars.
a. According to the model, when did the debt pass $1 trillion ($1000 billion)?
b. According to the model, what is the annual growth rate of the national debt?
c. What is the doubling time for this growth model?