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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

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