An experimental serum was injected into 500 guinea pigs. initially, 50 of the guinea pigs had circular cells. 200 had elliptical cells, and 250 had irregular cells. after the serum was injected, 0 of the guinea pigs with circular cells were affected, 125 with elliptical cells were affected, and 125 of those with irregular cells affected. find the empirical probability that a guinea pig with elliptical cells by injection of the serum. the probability is