c = number of children tickets sold
a = number of adult tickets sold.
we know that 178 total tickets were sold, thus whatever "c" and "a" are, c + a = 178.
the price of a child ticket is $5.4, so the price for all "c" tickets must be 5.4c.
the price of a adult ticket is $9, so the price for all "a" tickets must be ac.
we also know the total sales for all tickets combined is $1310.4, so then 5.4c + 9a = 1310.4.
![\bf \begin{cases} c+a=178\implies \boxed{c}=178-c\\ 5.4c+9a=1310.4\\[-0.5em] \hrulefill\\ 5.4c+9\left( \boxed{178-c} \right)=1310.4 \end{cases} \\\\\\ 5.4c+1602-9c=1310.4\implies -3.6c+1602=1310.4\implies -3.6c=-291.6 \\\\\\ c=\cfrac{-291.6}{3.6}\implies c=81](https://img.qammunity.org/2020/formulas/mathematics/middle-school/hb368hrnjquu5rl843tc7s6hgnrbe82y56.png)