You cannot base a real GMAT score off of a diagnostic test. That would be like saying "I failed algebra in high school, so I will never do well in math." Well, I did fail algebra in high school and never got anything below a "B" in a math class after that. You could get a 300 on the diagnostic test and then score a 740 on the real one just as you could get a 300 on the diagnostic test and then score a 300 on the real one.
1. Look at the problems you missed. Were they easy? Why did you miss them? If there were problems that you should have gotten correct, why did you miss them? If you missed them because of timing, because you forgot a formula, or because they were tough, no worries. If you missed easy problems simply because you could not understand them, you might want to start with a simple quantitative book, such as Foundations of GMAT Math: GMAT Strategy Supplement
by Manhattan GMAT
. Skip verbal. Unless you got a 0% in RC, everything can be fixed through prep.
2. Work through the MGMAT guides
in order. Complete the assigned OG problems (you should have OG 13
, the 2nd edition blue book, and the 2nd edition green book). For RC, start reading fiction books. Have a look at this thread
. When you are done, complete a GMATPrep test.
3. If you bombed the GMATPrep test, you will need extra study time. If you got a 650+, you will just need to brush up on weaknesses. Look at the problems you missed. If you do not understand them, post them in the quantitative section of GMAT Club. GMAT Club has some very intelligent members who are good at explaining even the most difficult problems.
4. Take the GMAT and score well!
This should take you two to three months, depending on how quickly you absorb the material.
I hope that helps
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