9514 1404 393
Answer:
 21
Explanation:
Let c represent the number of child tickets sold. Then 2c is the number of adult tickets sold. The total revenue is ...
 5.80c +9.50(2c) = 520.80
 24.80c = 520.80 . . . . . . . . . simplify
 c = 21 . . . . . . . . . . . . . . divide by 24.80
21 child tickets were sold that day.