The answer is 1600m2 assuming the side of the barn is at least 40 meters long.
An enclosure is usually a parallelogram and since you already have one side you need to make three more. If you dive 120m by 3 you get 40m. Each wall will be 40m. The enclosure ends up being a square and a formula for a square is base times height, therefore the dimensions of the enclore would be 40m times 40m which gives you the answer of 1600m2.
