Let's establish our unknown numbers with the variables x and y:
x amount of students
y amount of parents
We can establish these equasions:
x + y = 183 (number of students + number of parents)
2.50x + 4.25y = 538 (the amount each group paid)
To make things easier, we can put x or y from the first equasion on to the right side of that same equasion. I switched x in my case

We insert that into our second equasion and solve it

We now know that x = 137. If the total is 183, we just have to subtract 137 from that number to find out the value of y

RESULT:
x = 137 ==> 137 student tickets sold
y = 46 ==> 46 adult tickets sold