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There are two variables:
Let x = the number of $6 tickets
Let y = the number of $9 tickets
You need two equations.
Total sales were $10,500, so
Equation 1: 6x +9y = 10500
There were 250 more student tickets sold than nonstudent tickets, so
Equation 2: x = y + 250
Now take x=y+250 from Equation 2 and substitute it into Equation 1:
6(y+250) +9y = 10500
6y+1500 +9y = 10500
15y = 10500-1500
Now find x:
There were 850 student tickets and 600 nonstudent tickets.