a + s = 775
s = a + 75 ; where a = adult tickets and s = student tickets
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Solve for s in the first equation. Subtract a from both sides.
s = 775 - a
Plug s into the second equation.
(775 - a) = a + 75
Add a to both sides.
775 = 2a + 75
Subtract 75 from both sides.
700 = 2a
Divide both sides by 2.
a = 350
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350 adult tickets were sold.