GMAT Strategy

Hi,

I need a bit of help. I have just finished all the exercises in the Official Guide 12th edition. I used the OG error log kindly provided by beat the gmat team. When I analyzed my overall results, I found a very interesting point:

-My performance on the verbal section is:
86% correct answers on the easy questions
76% correct answers on the medium questions
71% correct answers on the hard questions

-My performance on the quant section is:
85% correct answers on the easy questions
70% correct answers on the medium questions
52% correct answers on the hard questions

As you can understand, my concern is on the hard questions of the quant section. If I want to achieve a high score, I would need to improve this ratio. I understand that the way GMAC calculates the GMAT scores depends more on the number of hard questions a test taker answers correctly. Therefore I presume if I were to improve my performance on the hard (and medium) questions of the quant section, I would be able to achieve a significant increase in my score.

Is there anyone out there who could recommend a good strategy to improve on hard quant questions both DS and PS? This could be a book/publication, a resource on the net, specific hard exercises or simply any other advice. I am planning to take the GMAT in 3/4 weeks time and I have been studying for a month or so.

Many thanks,