Unit 21 Section 1 : Introduction to Probability

A probability describes mathematically how likely it is that something will happen.

We can talk about the probability that it will rain tomorrow or the probability that England will win their next football match. We often use words like certain, likely, unlikely or impossible to describe the chance of something happening.

Example Question
Look at the three events below. Decide which of the words certain, likely, unlikely or impossible best describes how likely each event is, then click on the button marked Click on this button below to see the correct answer to see whether you are correct.

(a) Your maths teacher is older than you.

(b) Exeter City FC will win the next FA Cup.

(c) You obtain a seven when rolling a normal dice.

(d) It will rain somewhere in England during the next week.

The Probability Line
We can represent probabilities on a number line, as shown in the diagram below.

An unlikely event would have a probability between 0 and .
A likely event would have a probability between and 1.

Example Question
Look at the probability line below. The probabilities of the events A, B, and C have been marked on the line.

Look at the descriptions of three events below and match them up with the letters A, B and C on the line above
Once you have decided which letter matches with each event, click Click on this button below to see the correct answer to see whether you are correct.

(a) You roll an unbiased dice and get an even number.

(b) Christmas Day is in the next twelve months.

(c) You pick a card from a normal pack of cards and get an ace.

 

Exercises

Work out the answers to the questions below and fill in the boxes. Click on the Click this button to see if you are correct button to find out whether you have answered correctly. If you are right then will appear and you should move on to the next question. If appears then your answer is wrong. Click on to clear your original answer and have another go. If you can't work out the right answer then click on Click on this button to see the correct answer to see the answer.

Question 1
Use one of the words certain, likely, unlikely or impossible to complete the sentence describing each event below.

(a) It is that a pupil chosen from your class will be more than two years old.

(b) It is that you will obtain a six when you roll a normal dice.

(c) It is that you will score 13 when you add the scores on two normal dice.

(d) It is that it will snow somewhere in England during January.

Question 2
The probability line below shows the probability of 5 events, A, B, C, D and E.

Use the letters for the events to complete these sentences.

(a) Event is certain.

(b) Event is impossible.

(c) Event is unlikely but still possible.

(d) Event is most likely, but not certain to occur.

Question 3
The probability line below shows 5 different probabilities.

Look at the 5 events below and match them up with the probabilities on the line above.

(a) The probability of picking a "picture card" from a normal pack of cards is represented by

(b) The probability of the sun rising tomorrow is represented by

(c) The probability of getting "tails" when you flip an unbiased coin is represented by

(d) The probability of getting a number less than 10 when you roll a dice is represented by

(e) The probability of you travelling to school by plane tomorrow is represented by

Question 4
The events A, B, C and D have probabilities as shown on this probability line:

(a) Which event is most likely to take place?

(b) Which event is most unlikely to take place?

(c) Which events are more likely to occur than event B?

(d) Which events are less likely to take place than event A?


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Produced by A.J.Reynolds January 2001