A box contains purple discs and orange discs.
The ratio of purple discs to orange discs is 5:15:15:1.
Two discs are picked at random from the box.
The probability that both discs are orange is 166\displaystyle \frac{1}{66}661.
Work out the total number of discs in the box before any were picked.
43 exam-style questions on OCR GCSE Maths Probability Equation Questions. Each one has a worked solution and a mark scheme showing where the marks go.