Answer:
70 yards
Step-by-step explanation:
we know that
The perimeter of the rectangular pen is equal to
[tex]P=2(L+W)[/tex]
we have
[tex]P=160\ yd[/tex]
The width is between 10 and 20 yards inclusive
we know that
The longest pen will be for the smaller value of W
so
For W=10 yd
substitute in the equation of perimeter
[tex]160=2(L+10)[/tex]
solve for L
[tex]L+10=80\\L=80-10\\L=70\ yd[/tex]