Answer:
19 ft
Step-by-step explanation:
The perimeter of a rectangle is given by ...
P = 2(L +W)
Filling in the given values, we have ...
78 = 2(20 +W)
39 = 20 +W . . . . . divide by 2
19 = W . . . . . . . . subtract 20
The width of the rectangular pen is 19 feet.
