There are 3 doors before mr. A . he is told that one of them has a great prize while the other 2 would be nightmares for him. He chooses a door and suddenly one of the other 2 doors is opened and an ugly old PC is seen . There is a chance near Mr.A to change his door . if the probability of prize behind the door he choose after this is x and probability that it is behind the other door is y. Then 6(y-x) is?