Beat The GMAT a GMAT & MBA community

I have 40 more days to go before my G-day. I am generally good in qunta and score consistently good in OGs but what I have heard on this and other forums is that OG is not best of the guides if you are tragetting 50+ score. I want to practice questions specifically on the topics of -

Permutation and Combination

Probability
Standard deviation
Line Coordinates (y=mx+c related)
Any other toughie topics
Please suggest the best source that can help me brush-up these topics quickly and achieve my target of 50+.

_________________
A falling tree resounds... but a forest grows in silence...