How do I apply the correct formulae and other methods to difficult looking questions?

The trick here is to figure out what topic the question relates to, and recall all relevant theory and formula. Often questions look tougher than they actually are. Begin by writing out what you know at the beginning of the question and work step by step applying all the relevant information and numbers to what you're attempting to solve. Many questions involve a few steps but with practise, you'll soon get to grips with how to solve more difficult questions. Maths is an acquired subject, and you'll learn by doing. (Would then go through questions with the student)