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3a + 4c = 80,   (1)
2a + 3c = 57.   (2)
Let us solve it using the Elimination method.
Multiply eq(1) by 2 (both sides. Multiply eq(2) by 3. You will get
6a + 8c = 160,   (3)
6a + 9c = 171.   (4)
Now subtract eq(3) from eq(4). You will get
c = 171 - 160 = 11.
Thus the children ticket costs $11.
Then from eq(2), the adult tickets costs  
 = $12.
Please check it on your own.