Answer
The federal government has to pay back about ${{\$}} 1019$ billion.
Interest = ${{\$}} 232$ billion.
Work Step by Step
$r=0.013, P=787$ (billion),
$t=20, $ compounded $n=2 $ times a year:
$A=P\displaystyle \cdot(1+\frac{r}{n})^{nt}=787(1+\frac{0.013}{2})^{2\cdot 20}\approx{{\$}} 1019$ billion
$I= A-P=1019-787={{\$}} 232$ billion.
The federal government has to pay back about ${{\$}} 1019$ billion.
Interest = ${{\$}} 232$ billion.
