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Let x = # of $8 tickets and y = # of $12 tickets.
Then the number of $6 tickets is (x+y+280).
The "items" equation is
(x + y + 280) + x + y = 840, (1)
The "value" equation is
6(x+y+280) + 8x + 12y = 6080. (2)
Simplify
2x + 2y = 560 (560 = 840-280)
14x + 18y = 4400 (4400 = 6080-6*280)
Or even better
x + y = 280
7x + 9y = 2200.
Solve by any method you know (substitution, elimination).