Step 8 Objective

Know that the sum of probabilities of all mutually exclusive outcomes is 1, and use this when solving problems.

Examples of what pupils should know and be able to do

Arriving at the traffic lights at Hall Lane I know that the chance they are red is 0.4 and the chance they are green is 0.5. What is the chance they are on amber?

In an experiment find a missing probability and therefore a missing frequency.

Probing questions

Why is it helpful to know that the sum of probabilities of all mutually exclusive events is 1?

Give me an example of how you have used this when working on probability problems.

What if pupils find this a barrier?

Use a set of cards numbered from 1 to 20. Form two sets and identify the probability of picking each card from its set. Explore the link with the probability of choosing one of the remaining cards.