I was going to make a complete lesson on the Petersen Capture-Recapture method, then came across this one by TES user dannytheref who tweets as @mrburkemaths. I'm not going to be able to do better than this, so please make sure you give him plenty of lovely reviews!

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Below are some great exam questions from Emporium Maths @Edexcel with a corresponding mark scheme for a bit more practice: