If you feel that practice test was a fluke, take another one (a free MGMAT test, the other GMATPrep test, etc.). Use the results from that test to identify the places that are hurting you the most and attempt to improve.
15 days is not very much time.
...and yes, many people have stated the MGMAT books
were very useful.
Please consult the math book review thread:best-gmat-math-prep-books-reviews-recommendations-77291.html
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