Levels 2, 3 and 5 are meant to simulate what you would learn from GCSE and A level textbooks. You should study it more formally alongside this if you go through the documents and don't feel you have done it in school or basically know it. Proofs for some things in here are deferred to levels 4 (for most things) and 6 (for level 3.3 and particularly hard things). In many textbooks some of the proofs given in this level are provided but are oversimplified and not always fully correct (I point out common pitfalls as I go).
Open document for level 3.1 (General mathematics part 1)