The perimeter P of a rectangular livestock pen is 40 feet. If the width w is 6 feet, find the length.
First we substitute 40 for P and 6 for w in the formula P=2 l+2 w. Then we solve for l:
\begin{array}{lrl}\text { When } & P & =40 \text { and } w=6 \\\text { the formula } & P & =2 l+2 w \\\text { becomes } & 40 & =2 l+2(6) \\\text { or } & 40 & =2 l+12 \quad \text { Multiply } 2 \text { and } 6\end{array}\begin{array}{ll}28=2 l & \text { Add }-12 \text { to each side } \\14=l & \text { Multiply each side by } \frac{1}{2}\end{array}
To summarize our results, if a rectangular pen has a perimeter of 40 feet and a width of 6 feet, then the length must be 14 feet.