gmat prep probability

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vinviper1: Master | Next Rank: 500 Posts; Posts: 222; Joined: Tue Mar 25, 2008 3:52 pm; Thanked: 2 times

by vinviper1 » Thu Aug 28, 2008 10:58 am

Is there a quicker way to do this than the actual calculation? Thanks.

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Source: Beat The GMAT — Problem Solving | Thu Aug 28, 2008 10:58 am

somail: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Tue Jul 01, 2008 11:54 am; Location: San Diego

by somail » Thu Aug 28, 2008 11:28 am

NOt really, but you can quicken things by cancelling: For example:

18!/(4!*14!)=

18*17*16*15*14*13*12*11*10*9*8*7*6*5*4*3*2*1
_____________________________________________
(4*3*2*1) * (14*13*12*1*/10*9*8*7*6*5*4*3*2*1)

Notice how you can cancell out a large portion of the top number, so you get:

18*17*16*15
___________
4*3*2*1

73,440/24=3,060

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Thu Aug 28, 2008 1:51 pm

somail wrote:
18*17*16*15
___________
4*3*2*1

73,440/24=3,060

To this fraction, you could apply your very good advice:

somail wrote:NOt really, but you can quicken things by cancelling

For example, you can cancel the 4 with the 16, the 3 with the 15, and the 2 with the 18 to get:

9*17*4*5

Now notice how far apart the answer choices are; estimation will easily get us the answer. As long as you can see quickly that this number is a lot larger than 1260, you don't need to do this (rather annoying) multiplication in full. 9*17*4*5 is not far from 10*20*4*5 = 4000, so E must be the right answer.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

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