Mr. K wants to build a pen in his yard for his dog. He buys 1200 ft of fencing. He concludes that he can only build a rectangle with sides of 400ft and 200ft. Is he correct? If he is incorrect give a counter example.

I need help finding the answer.

He could build a square pen with sides of 300 feet each.

He could also build a long pen with sides of 100 and 500 feet.

There are many other possibilities such as 50 by 550, and so on.