We know that
• 9 tickets cost $54.
,• Adult tickets cost $8.
,• Child ticket costs $3.50.
Let's call x adult tickets and y child tickets.
Using the given information, we can define the following.
[tex]x+y=9[/tex]Because Jose bought 9 tickets in total.
Then, we express another equation.
[tex]8x+3.50y=54[/tex]This equation represents the costs.
Let's isolate y in the first equation.
[tex]y=9-x[/tex]Then, we replace this last expression in the second equation.
[tex]8x+3.50(9-x)=54[/tex]We solve for x.
[tex]\begin{gathered} 8x+31.50-3.50x=54 \\ 4.50x=54-31.50 \\ 4.50x=22.50 \\ x=\frac{22.50}{4.50} \\ x=5 \end{gathered}[/tex]