How to recognise and make the link between probability and the algebraic demands of this question?

While I doubt students will ask this exact question, think it provides the basis for what are usually the most difficult questions in GCSE papers, and students would usually be unsure where to start. My inspiration just stems from an Edexcel Question which went viral a couple of years ago. (Would insert question on whiteboard program)1.I would list down all the information available from the question.2. So we need to get to an equation involving n, so I would then represent each piece of info in terms of n.3.Create the equation involving n.4.Then rearrange for the answer.