Answer:
81 adult tickets
Explanation:
Step 1:
Set up a system of equations
Since the adult tickets and student tickets add to 243, we can do

and the student tickets sold were 2x the adult tickets, we can do

We can replace s in the first equation adding to 243 with 2a
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Step 2:
Solve for a. We can divide both sides by 3.

Therefore, there were 81 adult tickets sold.
Step 3:
We can check our work! Since we learned earlier the student tickets are 2x adult tickets, we can multiply 81 by 2 and add that to 81 to see if it equals 243!

And...

So, 81 must be the answer!