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Let x be the number of adults.
Then the number of children is 2x, according to the condition,
and the number of students is the rest (315-x-2x) = (315-3x).
The "money" equation (the revenue equation) is
12x + 5*(2x) + 7*(315-3x) = 2282.
12x + 10x + 7*315 - 21x = 2282
x = 2282 - 7*315 = 77 is the number od adults.
ANSWER. 77 adults; 2*77 = 154 children and the rest (315-77-154) = 84 students.
Solved.
The lesson to learn from the solution is THIS :
This problem is to be solved using ONE unknown - not three.