Probability Cambridge IGCSE Mathematics (9–1) revision
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In plain words
Probability puts a number on how likely something is, from 0 (it can't happen) to 1 (it's certain). If all the outcomes are equally likely, it's just the number of ways of getting what you want, divided by the number of things that could happen.
Three things to know
- Probability = number of favourable outcomes ÷ total number of outcomes. It is always between 0 and 1.
- The probabilities of everything that could happen add up to 1. So P(not A) = 1 − P(A).
- Relative frequency is a probability estimated from an experiment: successes ÷ trials. Expected number = probability × number of trials.
Worked example
A bag holds 3 red, 5 blue and 2 green counters. One is picked at random. Find the probability that it is not blue.
- There are 10 counters, and 5 are blue: P(blue) = 5/10 = 1/2.
- P(not blue) = 1 − 1/2.
- = 1/2.
Tips and tricks
- Write probabilities as fractions, decimals or percentages. Never as "1 in 4" or "1 : 4".
- More trials make a relative frequency a better estimate.
It lands in your notebook with its questions as flashcards.
Probability: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
A fair coin is tossed. What is the probability of a head?
One outcome out of two.
The probability of winning a game is 0.3. What is the probability of not winning?
1 − 0.3.
A bag has 4 red and 6 blue balls. What is the probability of picking a red one?
4 out of 10.
A dice is rolled 60 times. How many sixes would you expect?
1/6 × 60.
What is the probability of something that is certain to happen?
Probabilities can't be more than 1.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Probability
Cambridge IGCSE Mathematics (9–1) 0980 · 6 marks · papermunch.org
Name ______________________________ Date ______________
A fair dice is rolled. Find the probability of getting a number greater than 4.[2]
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1/3. Two outcomes (5 and 6) out of six.
The probability that it rains tomorrow is 0.35. Find the probability that it does not rain.[2]
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0.65.
A spinner lands on red with probability 0.2. It is spun 150 times. How many times would you expect it to land on red?[2]
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30. 0.2 × 150.
Answers: Probability
- 1. 1/3. Two outcomes (5 and 6) out of six.
- 2. 0.65.
- 3. 30. 0.2 × 150.