A firm can hire as much labor as it wishes at a fixed wage W. The change in variable cost is the per-unit cost of the extra labor w times the amount of extra labor needed to produce the extra output△L. Since△VC=w△L,it follows that MC=△VC/△Q=w△L/△Q Copyright by Caidonghong Copyright by Caidonghong ❑ A firm can hire as much labor as it wishes at a fixed wage W. The change in variable cost is the per-unit cost of the extra labor w times the amount of extra labor needed to produce the extra output △L. Since △VC= w△L, it follows that MC = △VC/△Q = w△L/△Q