You all know what a Fermi Problem is, even if - as a dev - you didn't know that was what they are called. They are those annoying questions you get asked at interview, you know, the ones that go "So, can you tell me how many filling stations there are in the UK?". The ones where you think, "What a pointless question". Well, the fact is - to a data scientist - they are extremely important in predictive analytics and in this session, we'll lift the lid on Fermi Problems and I'll show you exactly where, and why, they are useful.