I recommend buying the Schaum's College Algebra book. It is a great math reference book that summarizes a ton of math rules with examples.
I would use this book as a supplement to your practice DAT math problems. For example, if you get a problem that requires you to figure the square root of 3.6X10^-5, and you don't quite understand the exponent rules, the Schaum's book will give you a refresher. By the way the answer is sqrt(36)*sqrt(10^-6) = 6X10^-3 = 0.006.
I would do a bunch of practice problems, and then read the explanations to the ones you got wrong. If you are a little confused about the explanation, then refer to the Schaum's book.
Lastly, if a month goes by, and you are still having problems breaking 15 on QR on timed tests, then I recommend getting a math tutor through Craig's List or through your school.
Hope this helps.