You are right. Big Bang, the questions 1 - 5 in the HSC for Ext1 are usually those type of questions, not so much 4 and 5 but 1 through 3, it is incredible easy to make silly mistakes even if you have practised them 10000 times, and yes you have to show your working out, even for the simplest questions, if the criteria asks specifically for working out, even for a one mark question you must do it. Even now doing Ext2, the first few questions are much like the "easy questions" but you almost certainly have to put in your working out. You should be doing it anyway, it helps you focus and keep track on silly mistakes (even on the really really easy ones). It shows the marker you are thinking