[tex]\begin{gathered} let\text{ x=number of adults tickets, and y=number of child tickets} \\ \text{ Since three times as many adult tickets as child tickets were sold, then} \\ x=3y \\ \text{ since the total sales were \$688, then} \\ 9.20x+5.80y=668 \\ \text{ But we know x=3y,. so} \\ \\ 9.20(3y)+5.80y=668 \\ 27.6y+5.8y=668 \\ 33.4y=668 \\ y=\frac{668}{33.4} \\ y=20 \\ \\ \text{Thus the number of chld tickets were sold that day is 20, and the adult's 60} \end{gathered}[/tex]
