Assume that the U.S. real GDP is $13 trillion and grows at a 3 percent annual growth

Question:

Assume that the U.S. real GDP is $13 trillion and grows at a 3 percent annual growth rate over the next several decades. How long would it take the economy to double in size? How long would it take for the economy to double in size if the growth rate is 2 percent? Why can a lower growth rate be considered an economic waste?