Persons entering a blood bank are such that 1 in 3 have type O+ blood and 1 in 15 have type O- blood. Consider three randomly selected donors for the blood bank. Let X denote the number of donors with type O+ blood and Y denote the number with type O- blood. Find the probability distributions for X and Y. Also find the probability distribution for X + Y , the number of donors who have type O blood.