Let a represents adult and c represents child;
On Wednesday;
[tex]\begin{gathered} a=2c \\ 9.10a+5.10c=605.80 \end{gathered}[/tex]Then, we substitute the value of a in the equation, we have;
[tex]\begin{gathered} 9.10(2c)+5.10c=605.80 \\ 18.20c+5.10c=605.80 \\ 23.30c=605.80 \end{gathered}[/tex][tex]\begin{gathered} c=\frac{605.80}{23.30} \\ c=26 \end{gathered}[/tex]26 child tickets were sold that day.