uroojk wrote:how would you do 12 c 2 without a calculator quickly?

i am having trouble figuring this out..
i guess it would help if i could figure out how to factorials without a calculator quickly

i'm going to assume that you can expand the expression "12 choose 2", since you mention it so casually.

this expression expands to (12!) / (10! 2!).

from what you're typing, it appears that you may actually be trying to evaluate this expression by multiplying out the factorials (!!!!) and then performing some sort of long division on the resulting numbers (!!!!).
this is really not the way to go.

instead, you should realize the following major takeaway:
larger factorials will always "swallow" smaller factorials.

here's what i mean: factorials consist of products of ALL positive integers up to and including the given integer. therefore, all of the integers that appear in some smaller factorial will also appear as part of a larger factorial.

so, in the quotient above, all of the numbers from 1 through 10 (from 10!) will also appear in 12!. you can thus cancel those numbers, leaving only the 11 and 12 remaining from 12!.
therefore, the fraction equals (11 x 12) / (1 x 2), or 11 x 6 = 66.

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if you find yourself actually having to multiply out a factorial that is larger than, say, 5!, then you are probably missing out on LOTS of cancellation.
the gmat is not a test of "grinding arithmetic".

however, if you DON'T KNOW the shortcuts, then, by all means, you should try to grind out the computations. anything is better than sitting there staring at the problem.