The Solution:
Given:
[tex]\begin{gathered} L=3W \\ where\text{ L=length and W=width.} \end{gathered}[/tex]The perimeter of a surface of the rabbit pen must not be more than half of 240-feet
[tex]P=3W+3W+W+W\leq\frac{240}{2}[/tex][tex]\begin{gathered} 8W\leq120 \\ \\ W\leq\frac{120}{8} \\ \\ W\leq15ft \end{gathered}[/tex]The maximum length is:
[tex]\begin{gathered} L\leq3(15) \\ L\leq45ft \end{gathered}[/tex]