1. What is the probability that she has to stop exactly once?
Helen passes through 2 sets of traffic lights on her way to work.
The probability that she stops at the first set is 0.3
The probability that she stops at the second set is 0.2
What is the probability that she has to stop exactly once?
Lights 2
Lights 1
Stops
P(SS) = 0.3 x 0.2 =
0.06
0.2
Stops
Doesn’t
Stop
0.3
P(SD) = 0.3 x 0.8 =
0.24
0.8 OR
Stops
P(DS) = 0.7 x 0.2 = 0.14
0.2
0.7
Doesn’t
Stop
0.8
Doesn’t
Stop
P(DD) = 0.7 x 0.8 = 0.56
P(exactly once) =

2. mutually exclusive events
AND / OR
This AND This are independent events
we multiply the probabilities of independent events
AND = MULTIPLY
(This AND This) OR (That AND That) are
mutually exclusive events
we add the probabilities of mutually exclusive events
OR = ADD

3. Calculate the probability of picking one milk chocolate and one dark chocolate in any order
Second Pick
First Pick
(Milk AND dark) OR (dark AND milk)
= x x =
= 12 25
Milk
Milk
Dark OR
Milk
Dark
Dark

4. Classwork WS

5. Notes & Worksheets: €3.50

6. Homework – collect Tomorrow
STP 9 Pg 74 No. 5,7 and 9