There are some counters in a bag. Some of the counters are red. 3 of the counters are blue. 2 of the counters are green. Billie takes two counters at random from the bag. The probability that both of the counters Billie takes are the same colour is 14\displaystyle \frac{1}{4}41.
Work out the number of red counters in the bag.
You must show your working.
72 exam-style questions on OCR GCSE Maths Conditional Probability. Each one has a worked solution and a mark scheme showing where the marks go.