well, the subtotal or pre-tax bill is $23.50, now let's add the 8% tax and
[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{8\% of 23.50}}{\left( \cfrac{8}{100} \right)23.50}\implies 1.88[/tex]
so the post-tax bill is 23.50 + 1.88 = 25.38, and Jackie's tip is 20% of that
[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{20\% of 25.38}}{\left( \cfrac{20}{100} \right)25.38}\implies 5.076\qquad \approx 5.08[/tex]
