So far, we have looked at using tree diagrams to find the outcomes of multiple events when we are given the probability of a single event.
Now we need to look at doing this the other way round.
Example 1: A bag contains red and blue balls. A ball is picked out, the colour noted and then replaced. This is repeated.
If the probability of picking out two red balls is 1/25. What is the probability of picking out two blue balls.
Step 1: Lets put this into a tree diagram.
Step 2: What can we work out?
We know that the probabilities are the same for each event because we are told the ball is replaces.
If we know that P(AA) = 1/25, this is the same as saying P(A) x P(A) = 1.25 so P(A) = 1/5
Step 3: Work out the other event.
We know that A and B are exhaustive, so P(B) must equal 4/5
Step 4: Complete the rest of the tree diagram.
Step 5: Use this to answer the question.
We were initially asked for the Probability of 2 blue balls. From the tree diagram, we can now see that this is 16/25