If you would like to solve this problem, you can calculate this using the following two equations:
a ... the number of adult tickets
s ... the number of student tickets
$2905 = a * $8 + s * $3 ... 2905 = 8 * a + 3 * s
a + s = 560 ... a = 560 - s
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2905 = 8 * a + 3 * s
2905 = 8 * (560 - s) + 3 * s
2905 = 8 * 560 - 8 * s + 3 * s
2905 - 8 * 560 = - 5 * s
-1575 = - 5 * s /(-5)
s = 315
a = 560 - s = 560 - 315 = 245
Result: The box office sold 315 student tickets and 245 adult tickets.