Probability Trees: Theory The basic idea of a probability tree is to use the properties of probability to make a branching diagram of different outcomes.

Question A bag contains 4 blue counters and 3 red counters. A box contains 5 blue counters and 2 red counters. Complete the tree diagram and work out the probability of selecting two red counters.

First, represent the information on a tree diagram: From the tree diagram, the probability of winning the second set = 27 50 + 10 50 = 37 50.