well, we know is 10% for the first 7500, but Dan only made 5000 of taxable income, so he's in that range of 0 - 7500, so he only gets to pay 10% of 5000.
[tex]\begin{array}{|c|ll} \cline{1-1} \textit{a\% of b}\\ \cline{1-1} \\ \left( \cfrac{a}{100} \right)\cdot b \\\\ \cline{1-1} \end{array}~\hspace{5em}\stackrel{\textit{10\% of 5000}}{\left( \cfrac{10}{100} \right)5000}\implies 500[/tex]
