Suppose Shane wants to build a rectangular pen for his dogs to play in. He plans to use metal wire on three of the sides that will cost him $5 per foot and wood on the fourth side that will cost $10 per foot. He would like the pen to have an area of 600 ft2 . What are the dimensions of the pen that will minimize its cost? (Be sure to check you have found a minimum, which might not be at a critical point.)