Please explain how to solve this question. Supposed that all of the outcomes of a random variable are {a, b, c, d}, and that each outcome has an...

Please explain how to solve this question. Supposed that all of the outcomes of a random variable are {a, b, c, d}, and that each outcome has an equal 1/4 probability of occurring. Define the events A= {a, b, c}, B= {c, d}, and C= {d}. What is the probability of the event B given that the event A has occurred, P{B|A} ?