Question 1007837
Call the number of student and adult tickets, s and a.  Then we have
s + a = 20   and the value equation is
25s + 30a = 540
Now mutiply the top equation by 25 and subtract from the second equation...we get
25s + 30a = 540
-(25s + 25a = 500)
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5a = 40
a = 8 adult tickets
s = 12 student tickets