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) = 0.24 + 0.14

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

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) = 18 x 12 + 12 x 18 30 30 30 30 = 6 + 6 25 25 = 12 25 Milk Milk Dark OR Milk Dark Dark

Classwork WS

Notes & Worksheets: €3.50

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