You sell tickets for admission for your school play and collect $104. admission is $6 for adults and $4 for children. you sold 21 tickets. how many adult tickets and children tickets did you sell?

A + c = 21a = 21 - c
6a + 4c = 104

6(21 - c) + 4c = 104
126 - 6c + 4c = 104
-6c + 4c = 104 - 126
-2c = - 22
c = -22/-2
c = 11 <=== u sold 11 children's tickets

a + c = 21
a + 11 = 21
a = 21 - 11
a = 10 <=== u sold 10 adult tickets