1. Tree Diagrams
Be able to use a tree diagram to list the possible outcomes from two events.
Be able to calculate probabilities from tree diagrams.
Understand that Mutually Exclusive Events have probabilities which sum to 1.

2. The probability that it will rain on Monday is 0. 2
The probability that it will rain on Monday is 0.2. The Probability it will rain on Tuesday is 0.3.
What is the probability that it will rain on Monday and Tuesday?

3. We can solve this problem by drawing a tree diagram.
Tuesday
Look! Vertically, the numbers add up to 1.
It rains
0.3
Monday
We can solve this problem by drawing a tree diagram.
We know that the probability is 0.2
It rains
Look! Vertically, the numbers add up to 1.
0.2
There are two possible events here; It rains or It does not rain
0.7
It does not rain
Now let’s look at Tuesday
The probability that it rains on Tuesday was given to us as 0.3. We can work out that the probability of it not raining has to be 0.7, because they have to add up to 1.
Look! Vertically, the numbers add up to 1.
It rains
0.3
0.8
It does not rain
Well, raining and not raining are Mutually Exclusive Events. So their probabilities have to add up to 1.
What is the probability?
1 – 0.2 = 0.8
We now have the required Tree Diagram.
0.7
It does not rain

4. So the probability that it rains on Monday and Tuesday is 0.06
We wanted to know the probability that it rained on Monday and Tuesday.
Monday
It rains
It does not rain
Tuesday
0.3
0.7
0.2
0.8
0.2 x 0.3 = 0.06
This is the only path through the tree which gives us rain on both days
We can work out the probability of both events happening by multiplying the individual probabilities together
So the probability that it rains on Monday and Tuesday is 0.06

5. Actually, we can work out the probabilities of all the possible events
Monday
It rains
It does not rain
Tuesday
0.3
0.7
0.2
0.8
0.2 x 0.7 = 0.14
The probability of rain on Monday, but no rain on Tuesday is 0.14

6. Monday
It rains
It does not rain
Tuesday
0.3
0.7
0.2
0.8
0.8 x 0.3 = 0.24
The probability that it will not rain on Monday, but will rain on Tuesday is 0.24

7. Monday
It rains
It does not rain
Tuesday
0.3
0.7
0.2
0.8
0.8 x 0.7 = 0.56
The probability that it does not rain on both days is 0.56

8. Let’s look at the completed tree diagram
Monday
It rains
It does not rain
Tuesday
0.3
0.7
0.2
0.8
0.06
0.14
0.24
0.56
The end probabilities add up to 1. Remember this! It can help you check your answer!
What do you notice?

9. Time to have a go …
Try the questions from the book. Remember, if you are unsure, you can watch the presentation again at any point during the lesson.