Four balls are placed in a jar, two red, one blue, and one yellow. Two are removed at random.
1. You are told that the first ball removed was red. What is the probability that the second is red?
2. You are told that at least one of the two removed is red. What is the probability that both are red?
3. As in Exercise 17, but the first ball is replaced (but remembered) before the second ball is drawn. What is the probability that the second is red?
4. As in Exercise 18, but the first ball is replaced (but remembered) before the second ball is drawn. What is the probability that both are red?