Try to answer each question that you are required to. A partial answer is better than none. It is sometimes said that the first fifty percent of the marks for a question are easier to obtain than the second, meaning that if you can show that you understand the theory or concept involved you may receive a substantial proportion of the marks, even if you haven't the time to work out all the calculations. However, always make sure you show any steps and calculations you have done in a question, so that the marker can see how you have arrived at your answers. Read each question carefully.
Anxiety can cause carelessness.
One useful idea is to underline key words in the question as you read it through. Especially take note of exactly what is required from you, so that you do not prepare a solution or answer that is not quite what is being asked for. Some people just note that a question is about a certain topic or deals with a certain kind of situation, and then they rush ahead and write down all kinds of ideas they have about the subject or about that kind of problem. Don't do that. You're expected to think, not just react with an instant answer.