Answer
The width is $12\;feet$
and length is $36\;feet$.
Work Step by Step
Let the width is $w=x$.
Length is $l=3x$.
Total length of the fencing $P=96\;feet$.
Perimeter of the rectangular pen is
$\Rightarrow P=2(w+l)$
Substitute all the values.
$\Rightarrow 96=2(x+3x)$
Simplify.
$\Rightarrow 96=2(4x)$
$\Rightarrow 96=8x$
Divide each side by $8$.
$\Rightarrow \frac{96}{8}=\frac{8x}{8}$
Simplify.
$\Rightarrow 12=x$
Hence, the width is $w=12\;feet$ and length is $l=3x=3(12)=36\;feet$.
