.
With the given input data, we can write only one equation for two unknown "a" and "c".
This equation is the revenue equation
18a + 16c = 3600 dollars.
Having this equation only is not enough to solve the problem and to determine the unknowns.
For it, the second equation is necessary.
Usually, the second equation is for the total tickets sold;
but in this problem this total is not given; therefore, the second equation can not be attached.
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On tickets problems, see the lesson
- Using systems of equations to solve problems on tickets
explaining and showing different methods of solving such problems.
From this lesson, learn on how to solve such problems once and for all.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lesson is the part of this online textbook under the topic "Systems of two linear equations in two unknowns".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.