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1. October 4, 2012
Probability
1. Probability of an event not happening
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2. Revise October 4, 2012
Probabilities can be expressed as fractions, decimals or
percentages.
All probabilities lie on a scale of 0 to 1, or 0% to 100%.
Rolling a die, or any single occurrence for which we can
calculate the probability, is known as an event.
The sum of the probability of all events happening is 1.
Every event has an outcome, in this case, rolling the
number five is the outcome.
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3. Explanation October 4, 2012
Some things are more likely to happen than others.
You would be more likely to roll a five with a fair die,
than win the national lottery jackpot with a single ticket.
There are 6 equally likely outcomes when you roll a die.
The probability of rolling any one of these numbers is 1
6
There are 13,983,818 possible number
combinations in the national lottery draw.
The probability of having the winning
ticket is 1
13,983,818
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4. Explanation October 4, 2012
The probability that a particular outcome will happen can
be written in the shorthand P(outcome)
P(rolling a 5) = 1
6
Asking the probability of not rolling a 5, is the same as
asking the probability of rolling a 1 or 2 or 3 or 4 or 6.
P(not rolling a 5) = 5 or 1 – 1
6 6
so
The probability of an event not happening = 1– P
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5. Example October 4, 2012
There are 52 cards in a pack, in four suits, hearts, clubs,
diamonds and spades.
What is the probability of dealing out the 10 of spades?
The card is replaced and the pack shuffled. What is the
probability of dealing out a card that is not the 10 of
spades?
1
P=
52
1 51
P= 1– =
52 52
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6. Example October 4, 2012
The queen is replaced and the pack shuffled again.
What is the probability of not dealing a club?
There are 52 cards and 4 suits 52 ÷ 4 = 13
13 1
P(dealing a club) = 52 = 4
P(not dealing a club) = 1 – 1 = 3
4 4
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7. Examples October 4, 2012
The weather centre forecasts that there is a 60% chance
of it raining tomorrow.
What is the probability that it will not rain?
100% – 60% = 40%
Mikel decides to take a bus to school three days a week,
in an average week.
What is the probability that he did not do so today?
3 2
P(taking a bus to school) = 1–P=
5 5
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End