Adding a bit to what Miswimmy1 wrote, which I agree with:

If you are doing an exam with long (20 minutes or more) questions it is always worth spending a couple of minutes making dot-point notes about what you want to write before you start your answer (what you can write in 18 minutes is not very different what you can in 20 minutes).

This has two advantages: (1) It will help you to answer the question in a logical way and (2) It will help you to remember things about the question which did not immediately strike you as being relevant ("Doh, remember that X is relevant here.".

Both (1) and (2) are good things .