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.

Because Jose bought 9 tickets in total.
Then, we express another equation.

This equation represents the costs.
Let's isolate y in the first equation.

Then, we replace this last expression in the second equation.

We solve for x.

Therefore, Jose bought 5 adult tickets.