The only way the national debt could be paid back in just 8 years is President Trump were to tell those, who hold the debt, to stuff it and default.
Shhhhhhhhhhhh!!!!! Don't give him any ideas.
If you look at a pie chart of Federal spending, there is easily $1.5T per year that could be eliminated. $1T of that is welfare and we already know that if you insist people work for welfare, they opt out. That would pay off $12T in debt in 8 years.
If you look at past recessions and depressions, GDP growth in following years has been as high as 12%. If Trump’s policies could get just 6% for 5 years, tax revenues would have risen by $1.2T per year. Less in early years, more in later years, for a total of $8T additional revenue over the 8 years.
$20T easy peasy./s