Probability

1
Probability

• Understand the vocabulary and ideas of
  probability.
• Understand and use a probability scale.
• Calculate the probability of an event occurring.

2
Probability Vocabulary

• Put these words in order of likeliness

Unlikely
Impossible
Equally likely
Certain
Likely
3
Probability Vocabulary

• We put the words on a probability scale

Impossible
Equally likely
Certain
Unlikely
Likely
4
Where would you place these statements on a
probability scale?
Impossible Certain
A coin lands Heads side up
Next year will have 54 Fridays
The day after Monday will be Tuesday
You will watch TV tonight
It will rain in London in the first week of July
A new born baby will be a girl
5
What about these events?
Impossible Certain
You will be injured next time you play in a
sports game
A road accident will occur if the road is icy
A road accident will occur in normal driving
conditions
A cyclone will occur in England
A man will die before the age of 70
6
Rather than using words we use numbers on the
probability scale
Impossible Certain
0 0.5 1
The probability of an event occurring is written
as a fraction, a decimal or a percentage,
e.g.The probability of getting an even number
on rolling a dice is ½ or 0.5 or 50
7
How likely?

• To find the probability of an event occurring,
  work out how many things (called outcomes) could
  happen.
• Count how many of them give you the required
  event
• Write these numbers as a fraction Number of
  ways it happens Number of things that can
  happen

8
How likely?

• Number of ways it happensNumber of things that
  can happen

• The probability of getting an even number when a
  die (the singular of dice) is rolled
• P(even number) 3 6 (or ½ or 50 or 0.5)
• NEVER use ratios for probability.
• P(an event) means the probability of it
  happening.

9
Rolling a fair 6 sided die

• Calculate the probability of rolling
• a 5
• an odd number
• a prime number
• zero
• a number between 0 and 7
• a number greater than 2

1/6 3/6 ½ 3/6 ½ 0 1 4/6 2/3
10
Whats the probability that

• a letter chosen from all your names is a vowel?
• a pupil picked at random from the class is in
  your tutor group?
• a persons birthday is on a Sunday this year?
• a blue Smartie will be picked out at random from
  a tube?

11
From a normal pack of cards, whats the
probability that you choose
9/13 3/13 6/13 3/13 6/13 ¼ 1/26 1/52

• A number card (2 to 10)?
• A face card (jack, queen or king)?
• A prime number (let a Jack 11, King 13)?
• A square number (let the Ace 1)?
• Less than 7 (let the Ace 1)?
• A Diamond?
• A red Jack?
• The Queen of Spades?

12
Newspapers

• A newsagent delivers these papers one to each
  house
• Sun 250 Times 120
• Mirror 300 Mail 100
• Telegraph 150 Express 80
• What is the probability that a house picked at
  random has
• The Times?
• The Mail or the Express?
• Neither the Sun nor the Mirror?

120/1000 0.12
(10080)/1000 0.18
(1000-250-300)/1000 0.45
13
Roulette
P(odd number) P(1 to 10) P(Black) P(number 1)
15/30 ½ or 50 10/30 1/3 or 33 15/30
½ or 50 1/30 or 3.3