A bag contains 15 identical marbles. There are 7 red, 3 blue and 5 yellow marbles. A marble is picked out at random and not replaced. A second marble is then picked out at random and not replaced.
a) find the probability that
(i) both marbles are red
(ii) one marble is red and the other is not red
b) a third marble is now picked out at random. Calculate the probability that two marbles are red and one is not red.