Stick with it!!! Nav can be tricky at times....... I found bobs questions to be a little bit harder then the actually questions i got in the Cyber Exam. The Cyber Exam questions are very simliar to bob or even the same at times. Because it is multiple choice you can sometimes work backwards to check your answer!!!
It helped me to purchase and work through bob's practise exams online.......
http://www.bobtait.com.au/
Good Luck..