if all you're using is the OG, you definitely should pick up the MGMAT study guides found here:

https://www.manhattangmat.com/storeitems ... 28&catid=1

I'm not sure which book specifically has comb/perm but they will ALL be helpful in all aspects of the GMAT... these books go hand in hand with the OG as they reference certain problems in the OG pertaining to the topics in each chapter of the study guide. The best way to use these guides are to:

- go through each chapter... do the problem set at the end of the chapter (on a separate piece of paper)...
- check the answers (but do not read the solutions of the ones you got wrong) and circle the numbers that you had trouble with...
- then re-read the chapter (each chapter is anywhere from 2-6 pages i'd say) and see if you can do the questions you circled
- then go through the corresponding problems in the OG using the same process

I'm not sure how long you have until you take the GMAT exam, but I'm confident that if you follow this regimen you'll exceed 500. I did 1-2 chapters a night, because I need to get through the whole book in a week for my prep class. If you do 1-2 chapters a night you'll definitely beat 500!

Keep us updated!

p.s. - MEMORIZE THE GRAMMAR RULES FOR SC