I need to know this subject for my uni access exam, which will include one combinatorics problem. It's very silly because it's not studied in the course at all, you are instead given random problems and are supposed to magically know them and work them out in the most painful ways. I just don't have the physical time to waste it inventing formulas for how many lads shake hands (I wish I did because it's an interesting field) It's going to be something I'll read on my 'free' time after I've done my Maths, Physics and Chemistry work so it has to be as light of a read as possible. I'm not looking for a deep treatment with proofs. Definitions here and there and interesting problems is all I need.