a rancher needs to put a fence around a rectangular field that has one side along a river. The side along the river does not need a fence, while fencing for the side across from the river costs 20 dollars per foot, and the fencing for the other two sides costs 5 dollars per foot. Let x be the distance parallel to the river and let y be the other length. Find a formula (with x and y) for the total cost for the necessary fencing.