Unfortunately, math is not a subject in which you can do very well in a short time.
If you are very weak in math, as you state, then destroyer should not be the first book to use. You improve math through practice and more practice. You should also try to do the problems in your head rather than relying too much on your calculator.
For the first week or two, go grab a high school textbook for basic algebra and geometry skills and solve the problems listed in those books. Try to do the simple calculations in your head and confirm with a calculator.
For the rest, do the destroyer questions. You should also know that the destroyer questions are somewhat more difficult than the actual thing.
I'm not sure if you have enough time to improve on all of the weak sections in that short time, though.
