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 = 21 ... a = 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