1. Monty Hall
This is a old problem, but it illustrates the concept of conditional probability beautifully. References to this problem have been made in much popular culture, and a quick search on the Internet will reveal much information.
Set this game up with 2 people, and three cups and a prize hidden under the cups. Get one person to act as Monty. Monty knows where the prize is remember.
Play a set of games where you always stick and a set where you always change.
On a game show you are asked to choose one of three closed doors. Behind one is a car, behind another is a goat, and the other has nothing behind it.
Monty will open a door after you have picked, and then ask you if you want to stay with your choice or switch to the other unopened door. Of course he will never open the door you chose first.
What do you do - stick or change? Why?
What do you do - stick or change?

2. Constructing a tree diagram
Construct a tree diagram to show this information.
Tuesday
Rain
Monday
Rain
No rain
Rain
No rain
Keep the denominators the same in the final working.
No rain

3. Probabilities Calculate the probability that,
a) it rains at least once,
b) it rains one day only,
c) it rains on one day only, given it rains at least once.
It is possible to ignore the denominators here only if they are the same at the end of your tree diagram.

4. Questions
A teacher oversleeps with a probability of 0.3. If he oversleeps then the probability of him eating his breakfast is 0.2, otherwise it will be 0.6.
Misses
breakfast a)
0.8
0.24
Oversleeps
0.3
0.2
Eats
breakfast
0.06
0.4
Misses
breakfast
0.28
a) Construct a tree diagram to show this information.
0.7
Does not oversleep
Eats
breakfast
0.6
0.42
Use your diagram to find the probability that,
b) he oversleeps and does not eat breakfast,
c) he does not have breakfast,
d) he overslept, given he has breakfast,
e) he overslept, given he does not have breakfast. b)
0.24 c)
0.52 d) e)

5. Using tables French German Total Male 40 80 Female 90 30 120 130 70
Conditional probabilities can be found simply from data in tables, as illustrated by the following.
The table opposite shows the choices of language and the gender of the 200 students choosing those languages.
French
German
Total
Male 40 80
Female 90 30
120
130 70
200
d) being female, given he/she does French.
A student is choosing at random, find the probability of that student,
a) doing French,
e) doing German, given that he is male.
b) being male,
c) being male and doing German,