Mr.Jones bought 20 yards of fencing to make a pen for his dog, the pen will be in a retangle shape fencing comes in 1-yard sections,he want the biggest space possible for his dog to run. What are the dimensions of the maximum space of the pen he can get??

the dimensions must be integers, so the choices are below, given that adding the length and width must be half the perimeter, or 10.

1x9
2x8
3x7
4x6
5x5

which dimensions give the largest area (length x width)?