We have to find how many shirts (s) he has bought.
We know that each shirt costs $15, so the total cost of the shirts should be 15*s.
If we substract the discount, we have the total final cost C as:
[tex]C=15\cdot s-9[/tex]If we know that this total cost was $36, we can find the number of shirts as:
[tex]\begin{gathered} C=36 \\ 15s-9=36 \\ 15s=36+9 \\ 15s=45 \\ s=\frac{45}{15} \\ s=3 \end{gathered}[/tex]Answer: He bought 3 shirts (s = 3).
