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31 Jul 2008, 13:18
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How high should I know my factorials by memory? I've had problems where there's a 12! but that seems crazy to have to memorize!
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31 Jul 2008, 13:19
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The GMAT will require factorials in the context of Combination and Permutation problems. The way those formulas are set up to divide factorials before you actually calculate them.
For example:
8_C_3 = 8!/ [3!(8-3)!] = 8*7*6*5*4*3*2*1 / 3*2*1 * 5*4*3*2*1
Here the 5 through 1 factors in the numerator and denominator cancel to 1 leaving:
8*7*6 / 3*2*1
and the 3*2 in the denominator cancel the 6 factor in the numerator. That leaves simply 8*7, or 56.
I have lots of students who want to calculate 8! then divide it by 5! and 3! but canceling common factors in the numerator and denominator is WAY faster and less prone to error.
This is the manner in which factorials are tested on the GMAT.
For example:
8_C_3 = 8!/ [3!(8-3)!] = 8*7*6*5*4*3*2*1 / 3*2*1 * 5*4*3*2*1
Here the 5 through 1 factors in the numerator and denominator cancel to 1 leaving:
8*7*6 / 3*2*1
and the 3*2 in the denominator cancel the 6 factor in the numerator. That leaves simply 8*7, or 56.
I have lots of students who want to calculate 8! then divide it by 5! and 3! but canceling common factors in the numerator and denominator is WAY faster and less prone to error.
This is the manner in which factorials are tested on the GMAT.
31 Jul 2008, 13:21
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I think in working the factorials in prep for my GMAT i got to the point where I remembered that 7! is 5040. I never needed to know. Most of the time when you're dealing with factorials, it's a Comb/Perm problem and you can cancel many of the parts out. You'd want to write it on your paper like this in order to cancel out the top and bottom where you can
\(C_7^3 = \frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1} = \frac{7*5}{1}=35\)
\(C_7^3 = \frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1} = \frac{7*5}{1}=35\)
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
