Tree diagrams are useful to answer probability questions where there is more than one event happening in succession.

__Example__

There are 7 red balls and 5 yellow balls in a bag.

I take one ball from the bag and keep it.

I then take another ball from the bag.

What is the probability of choosing two red balls?

*(Give your answer as a reduced fraction in the form a/b)*

**Answer**

Here is a tree diagram to show this information.

Notice how the probabilities change for the second choice depending on whether the first choice resulted in a red or a yellow ball.

P(Red Red) = 7/12 x 6/11 = 42/132 = **7/22**