The school that Stefan goes to is selling tickets to a choral performance. On the first day of ticket sales the school sold 3 senior citizen tickets and 1 child ticket for a total of $38. The school took in $52 on the second day by selling 3 senior citizen tickets and 2 child tickets. Which system of equations can be used to determine s, the number of senior citizen tickets sold and c, the number of children tickets sold?

The correct answer is: 3s + 1c = 38
3s + 2c = 52

Now we know that one, two, or three variables may be present in a linear equation. As a result, we are able to formulate linear equations with n variables.
Here we have given that the school that Stefan goes to is selling tickets to a choral performance. On the first day of ticket sales the school sold 3 senior citizen tickets and 1 child ticket for a total of $38. The school took in $52 on the second day by selling 3 senior citizen tickets and 2 child tickets. 
So let number of senior citizen tickets be denoted as s, and c be the number of children tickets.

Then system of equations are,
3s + 1c = 38
3s + 2c = 52

So here we used the concept of system of equations in two variable. Here in the question the variable quantity was 2 so two variables are there. So the system of equations are: 3s + 1c = 38, 3s + 2c = 52