To be systematic you need two things in this exam. First, your hard work and motivation. Second, the way an examiner finds your answers. This is luck. An examiner might get the theme of your answer the way you intended it to be and he might offer you good marks. On the other hand he might be offended by one or two of your points and he may cross the question. So in a broader sense, to have a happy checker or an angry checker the moment your paper is to be checked, is luck my friend.