My maths teacher set this problem the other day: (Just clarifying that everyone knows how pool balls work, they are either stripey or solid colour) A bag contains one pool ball that is known to be a solid, a second ball is chosen in such a way that it has equal chance of being a solid or a stripe, this ball is then placed in the bag. A ball is now selected randomly from the bag and it turns out to be a solid. If somebody now selects a ball from the bag what is the probability of it being a stripe? I worked out an answer and i was told that it was wrong. I will post my answer shortly so as not to influence your decisions. Please could people have a go at this and post their reasoning.