GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 19 Jul 2018, 01:08

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# How high should I know my factorials by memory? I've had

Author Message
Intern
Joined: 31 Jul 2008
Posts: 2
How high should I know my factorials by memory? I've had [#permalink]

### Show Tags

31 Jul 2008, 13:18
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!

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Intern
Joined: 07 May 2008
Posts: 19

### Show Tags

31 Jul 2008, 13:19
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.
_________________

Ryan S.
| GMAT Instructor |
Elite GMAT Preparation and Admissions Consulting
http://www.VeritasPrep.com

SVP
Joined: 30 Apr 2008
Posts: 1841
Location: Oklahoma City
Schools: Hard Knocks

### Show Tags

31 Jul 2008, 13:21
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

$$C_7^3 = \frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1} = \frac{7*5}{1}=35$$

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________

------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a.

GMAT Club Premium Membership - big benefits and savings

Re: Factorials   [#permalink] 31 Jul 2008, 13:21
Display posts from previous: Sort by

# How high should I know my factorials by memory? I've had

Moderator: chetan2u

# Events & Promotions

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.