The cost function is
[tex]20x+300[/tex]where x is the number of wooden cases made
the revenue function is
[tex]30x[/tex]where x is the number of wooden cases sold
[tex]\begin{gathered} 30x=20x+300\rightarrow \\ 30x-20x=300\rightarrow \\ 10x=300 \\ x=\frac{300}{10}=30 \end{gathered}[/tex]so she has to make and sell 30 wooden cases to get her costs and revenue equals