Students have been up in arms about one of the questions on this summer's Edexcel GCSE questions.

There are n sweets in a bag. Six of the sweets are orange. The rest of the sweets are yellow. Hannah takes a random sweet from the bag. She eats the sweet. Hannah then takes at random another sweet from the bag. She eats the sweet. The probability that Hannah eats two orange sweets is 1/3. Show that n²-n-90=0.

The question is a little more abstract than the typical probability question, and includes a little bit of algebra. Step 1 is to set up your tree diagram. Note that because it's just asking you about 2 orange sweets, you don't need to fill out the other branches.

tree diagram

Now we know that we multiply along the branches to find the probability of both events occurring. Therefore, the maths is as follows:

tree maths

Not straightforward, but Edexcel has definitely included algebra in probability / tree diagram questions before. I suspect the main thing that has changed is that students can now use twitter to complain about it.