Hey Lolo,

That's a tough question and borders on some shaky ground...I don't think anyone can say for certain that you NEED to know those concepts to get a 650, but it's also not too hard to envision a test in which you see even 5-6 questions that test those concepts, in which case you'd be at a distinct disadvantage if you're just writing those off.

And I'd be really, really careful saying that you "only" need a 650. 650s are no joke - that has to be somewhere in the 75th-80th percentile, which means that only the top 20 or so percent of all GMAT test-takers score that high. Frankly, if you're behind enough in your studies or not confident enough in your abilities that you're willing to write off two concept areas, you probably shouldn't consider 650 to have an "only" in front of it. 650s are earned, not freely given away.

More productively for your study schedule, let me offer this:

1) Combinatorics and overlapping sets aren't "hard". Like any topic, they have the potential to be difficult, but at their most basic form they should be pretty self-evident, and there's a range in between. For example, an overlapping set problem like:

In a group of three high school friends, two are on the basketball team and two are on the soccer team. If each friend plays at least one of the sports, how many play both basketball and soccer?

Well, there are only 3 people but there are 4 team memberships between them. So one friend must be double-counted, and the answer is 1. This one doesn't even require much math at all, and certainly not that much pure knowledge of overlapping sets...it's a lot more logical.

Similarly, if they asked how many ways those three could allocate two tickets to a concert, that's technically a combinatorics problem, but it shouldn't require too much specialized knowledge. You could have A and B go; A and C go; or B and C go. So there are 3 different pairs that could go to the concert.


2) I'd highly recommend that you're able to think about those concepts even if you don't take the time to study specific formulas for them...if you see an average-level problem from those disciplines -and plenty of those exist - you're going to want to be able to do it. I'd also predict that if you've thought a little about those concepts at a fairly basic level, you'll have at least a fighting chance on the harder questions (on many of which you can use a fundamental understanding coupled with a number property or a good estimate to get the answer right without much work), and you'll be a much better GMAT thinker, able to solve seemingly-unique problems by reasoning your way through them.

3) Combinatorics is in many ways the basis for GMAT probability (which typically requires you to determine the number of total possible sequences - a permutation - as the denominator for a the probability of an occurrence), so again at least having a fundamental understanding of how to determine the number of sequences or arrangements is going to set you up for success on a good many problems.

4) Try to remember that the GMAT is testing your ability to think critically...it's not a measure of how many skills or concepts you've learned. So if you ever do see an unfamiliar concept, try to reason your way through it. Not only will that likely give you a pretty firm understanding of that concept, but you'll be practicing a valuable reasoning process that can help you on future such concepts.