Gilbert Greenfield owns a pasture on which the grass has been cut to uniform height of two inches. The grass in the pasture grows uniformly at a constant rate. In addition to owning the pasture, Mr. Greenfield also owns a cow, a horse, and a sheep. The grass existing and growing in the pasture is sufficient to feed all three animals grazing together for 20 days. It would feed the cow and the horse alone for 25 days; the cow and the sheep alone for 33⅓ days; and the horse and the sheep alone for 50 days. How long would it sustain (a) the cow alone? (b) The horse alone? (c) The sheep alone?