Number of co-primes to N, that are less than N

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hey_thr67: Master | Next Rank: 500 Posts; Posts: 299; Joined: Tue Feb 15, 2011 10:27 amFollowed by: 2 members; Thanked: 9 times

by hey_thr67 » Fri May 18, 2012 9:30 am

Can somebody explain why this number is N(1-1/a)(1-1/b)(1-1/c).... ?

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Source: Beat The GMAT — Problem Solving | Fri May 18, 2012 9:30 am

parveen110: Senior | Next Rank: 100 Posts; Posts: 91; Joined: Fri Jan 17, 2014 7:34 am; Thanked: 7 times

by parveen110 » Sat Feb 08, 2014 6:48 am

Even i was wondering this. I think it has something to do with progressions or combinatorics. Not sure. Experts please weigh in if there is a simple logic involved. Thanks!

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Sun Feb 09, 2014 11:23 am

Hi parveen110,

Is there a GMAT question attached to this concept that you'd like to review?

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Feb 09, 2014 3:49 pm

Rest assured that the concept of "coprime integers" (and this suspicious formula) is not required knowledge for the GMAT. Yes, it's conceivable that a GMAT question might test a certain aspect of coprimes, but if it did, the question would include a definition of "coprime."

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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