Thank you all for the advice, my original thought was that they might be able make progress in MA1 - using and applying. Maybe with an nrich activity but I'm not sure if that'd fit into 15minutes. Also not sure if for an interview they'll be expecting something original?? I also quite like the idea of giving the answer, they could discuss possible questions then I could reveal the operation or one of the numbers in the calculation so they could work missing numbers. Thanks again for all your suggestions, I will keep thinking about it!