A jar contains 6 red marbles, numbered 1 to 6, and 12 blue marbles numbered 1 to 12.
a. A marble is chosen at random. If you're told the marble is red, what is the probability that it has the number 5 on it?
b. The first marble is replaced, and another marble is chosen at random. If you're told the marble has the number 1 on it, what is the probability the marble is red?