Maths Topic 28 - Probability

Probability is a maths skill that is commonly tested in 11 plus exams and UK selective tests.

Probability can be tested in a variety of question types e.g. spinners, dice, balls in a bag, playing cards etc.

The answers may be requested as a fraction, percentage, type of chance (e.g. impossible, unlikely, even, likely and certain) or a ‘one in a number’ (e.g. 1 in 3 chance) chance.

If you are asked what the probability of randomly choosing a red ball is if there are 6 red balls and 4 blue balls in a bag, the answer could be given as a fraction (6/10), a percentage (60%), as a type of chance (‘likely’) or as a number (3 in 5 chance).

Examples

Example Question 1 (Dice):

What is the probability of rolling an even number, as a fraction?

Answer: 1/2

Explanation: There are 6 numbers on a dice and 3 of these are even numbers (2, 4 & 6) which is 3/6 as a fraction, this is simplified to 1/2.

Example Question 2 (Spinner):

There are 6 numbers on a spinner: 2, 2, 3, 3, 3 & 5. What percentage chance is there of spinning a prime number?

Answer: 100%

Explanation: All of the six numbers on the spinner are prime numbers.

Example Question 3 (Playing Cards):

There are 52 playing cards in a pack of cards, made from 4 different suits (hearts, clubs, diamonds & spades) which are in 2 different colours. What is the probability of randomly selecting a diamond card?

Answer: one in four chance

Explanation: There are 4 different suits and the diamonds are one of these four.