It is hard to answer your question without more information. Thus which version of Roberts and Hellers book?
You can read the objectives of that exam at
http://www.sun.com/training/catalog/courses/CX-310-035.xml What are you referring to when you say "most mock exams". I recommend you use resources that tell you what version of the exam they are aimed at and ideally include the actual text of the objective, e.g. Dan Chisholms material.