The question was:

*Hannah has 6 orange sweets and some yellow sweets.*

*Overall, she has n sweets. She takes one sweet from the bag and then another.*

*The probability of her taking 2 orange sweets is 1/3.*

*Prove that:*

The question is not actually that difficult if you remember how tree diagrams work.

When Hannah first takes a sweet there are 6 orange sweets out of n, so the probability of her choosing orange is 6/n.

When she chooses her second sweet there are now only 5 orange (if she chose orange the first time) out of a total of n-1 sweets.

You multiply the probabilities along the branches of a tree diagram, so

Multiplying the 2 fractions on the left gives:

Cross-multiplying gives:

I would doubt that the students had ever run through an equation like that in there GCSEs classes. The form of the equation is getting more into A level territory.

ReplyDeleteThis is the first correct explanation I have seen. And for all the whingers, it IS in the syllabus.

ReplyDeletehttp://justmaths.co.uk/wp-content/uploads/2014/06/GCSE_Maths_2015_Curriculum_Change.pdf