Unless your teacher tells you that you are doing mental maths, you should show all the working for your answers.
A well laid out answer can be read and understood by another person. This is an important skill to learn for the real world, not just for maths exams!
So don’t just write down numbers! Use words to explain the steps in your answers.
In the SQA exams you will only get full marks if you show full workings. Even if you make a mistake in an answer you may still get some marks for other parts of your answer. So it’s always best to show a full working.