I got that one wrong, too. But you have to be careful with the wording of that correct answer. I was thinking about the progressive income tax too, I should have read it more carefully. If you average everything out, the correct answer is indeed correct -- i.e. the sum of tax receipts/ the number of citizens (NOT "taxpayers") = total gov't spending/ the number of citizens (IF revenue is set equal to spending).
And, there are other ways the gov't incurs debt (e.g. foreign borrowing), so the answer you gave (which I also gave) isn't right.