OG questions are easier than the actual GMAT IMO. Well, except for the last 25 questions in the problems solving questions and last 75 questions in data sufficiency, the rest is not really tough. If you scored in mid 30s in PR, then you should have been fine to score within that range. I would perhaps recommend you the

Kaplan GMAT800 and the

Kaplan GMAT with CD. The math problems in

Kaplan are very challenging and it forces you to answer them fast under timed conditions. In all the prep material I found, except for

Manhattan GMAT, none of them really focused on combination/permutation problems. You can actually find many of those on this board. If you are aiming to score 40+ on the test, you will certainly see at least a few C/P problems. Last but not least, actively participate in solving questions on this site, it will sure help your quant skills. If ever you find any question challenging, please post it and see what other members will provide you as explanation to help you out. Good luck.

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Best Regards,

Paul