It depends on your confidence in the topic. If it is simple, you can do it directly. If you don't have a clear idea, you can make a rough calculation on the draft paper first, write your main idea on the draft paper (but try to be neat as possible, because the draft paper may not be enough), and then do the questions on the test paper. Sometimes, if the test paper is difficult, you can spend more time on the last question, whether you can do it or not.