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.