Sari is creating a dog run space in her backyard. She has 40 feet of fencing to go around the outside, and she wants to make it rectangular with a width of 8 feet. What will the length be, in feet?
____ feet
Let L be the length of the dog run space.
The perimeter is P = 2L + 2W = 40.
Substituting the given width, we get 2L + 2(8) = 40.
Combining like terms, we get 2L + 16 = 40.
Subtracting 16 from both sides of the equation, we get 2L = 24.
Dividing both sides by 2, we get L = 12.
The length will be 12 feet. Answer: \boxed{12}.