Micheal has 40ft of fencing for the rectangular dog run. So the perimeter of the dog run is 40ft.
Perimeter=40ft
The perimeter of a rectangle is:
P=2l+2w
Since we know the length is 5 and the perimeter is 40, we can solve for the width.
P=2l+2w
40=2*(5)+2w
40=10+2w
subtract 10 from both sides
30=2w
divide both sides by 2
15=w
The width of the fence is 15feet.