let a be the adult tickets and c be the child ticket.
1st equation: a + c = 165
2nd equation: 9.50a+ 5.30c = 1265.10
Solve the 1st equation for a:
a + c = 165
a = 165 -c
Substitute a value from 1st equation to 2nd equation:
9.50a + 5.30c = 1265.10
9.50(165 - c) + 5.30c = 1265.10
1567.5 - 9.5c + 5.30c = 1265.10
1567.5 - 1265.1 = 4.2c
302.4 = 4.2c
302.4/4.2 = c
72 = c
Answer: There were 72 child ticket sold.