Question 1130098
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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.


<U>ANSWER</U>.  77 adults;  2*77 = 154 children and the rest (315-77-154) = 84 students.
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Solved.


<U>The lesson to learn from the solution is THIS</U> :


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     This problem is to be solved using ONE unknown - not three.
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