He needs to pay back $64
He has already paid $4802, and according to the chart that goes with this question, he owed $4866.
What he owes: $4866
subtract what he has paid: $4802
So he needs to pay the IRS $64
He will pay $64
Using the attached image, for the head of household earning $36,558, he's bound to pay $4,866 considering that his age bracket is in the last line of the image ($36,550-$36,600). However, since he has already paid $4,802 then he hasn't paid it all. He remains with a balance of $4,866-$4,802=$64
Therefore, he's supposed to pay additional $64