That ignores money velocity. You don't have to find 105 unique currency units. Payment 1 can be 60, then after doing some work and getting paid you make another payment of 55. It doesn't matter at all that you reused some of the currency units.

Undisciplined

econ

They'll Never Pay Down the National Debt

Of course they can't pay it back. At a basic level, this is how our monetary system works:

I'm the Fed. To begin this new economy, I lend you (create) $100 on the expectation that you pay it back with 5% interest. So you owe me $105. Where are you going to get the extra $5 if only $100 exists?

You can't. You're going to have to borrow it. 