Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Home > Maths > Handling data > Probability




Sum of probabilities

If you toss a coin, the probability of obtaining a head is 1/2 and the probability of obtaining a tail is also 1/2.

Front and back of a 1p coin

P(head) + P(tail) = 1 / 2 + 1 / 2 = 1

If we choose a letter at random from the word 'SUMS', the probability of obtaining the letter 'S' is 2 / 4, the probability of obtaining the letter 'U' is 1 / 4 and the probability of obtaining the letter 'M' is 1 / 4.

P(S) + P(U) + P(M) = 2 / 4 + 1 / 4+ 1 / 4 = 1

Remember that the sum of the probabilities of all possible outcomes is 1


The probability that I am late for work tomorrow is 2/9. What is the probability that I am not late for work?


There are two possible outcomes - being late and not being late. The sum of their probabilities must add up to 1, so the probability of not being late is 1 - 2/9 = 7/9


Probability activity

What are the chances of you trying this activity?


Gut Instinct

Can you match Viral Vinnie's appetite for English, Maths and Science?

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.