Personal advice: Don't buy it.
Even if you do so, don't count on the scores. I paid and scored 580 and 590 on two of them right before I took the test and my test result appeared to be 540. I could understand 560-570 but 50 points gap is unbelievable. So it doesn't show you where you are. Plus, you don't have the explanations for the questions.
Go buy a
manhattan gmat book on a topic you experience problems in, and eventually you get 5 cat for free besides the book. You can also see the detailed explanations.
No offense mr. or mrs. representative but in my humble opinion gmat exam pack is waste of money.