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# The function f(m) is defined for all positive integers m as

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The function f(m) is defined for all positive integers m as [#permalink]

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26 Jan 2011, 13:00
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The function f(m) is defined for all positive integers m as the product of m + 4, m + 5, and m + 6. If n is a positive integer, then f(n) must be divisible by which one of the following numbers?

(A) 4
(B) 5
(C) 6
(D) 7
(E) 11
[Reveal] Spoiler: OA
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26 Jan 2011, 13:38
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PASSINGGMAT wrote:
The function f(m) is defined for all positive integers m as the product of m + 4, m + 5, and m + 6. If n is a positive integer, then f(n) must be divisible by which one of the following numbers?
(A) 4
(B) 5
(C) 6
(D) 7
(E) 11

Given: $$f(n)=(n+4)(n+5)(n+6)$$, where $$n$$ is a positive integer. Question: $$f(n)$$ must be divisible by which one of the following numbers.

Now, $$(n+4)(n+5)(n+6)$$ is the product of 3 consecutive integers so out of them one is definitely divisible by 3 and at least one is divisible by 2, so $$f(n)$$ must be divisible by 2*3=6.

Generally out of ANY $$k$$ consecutive integers one is always divisible by $$k$$ and at least one by $$k-1$$, $$k-2$$, ... For example out of ANY 5 consecutive integers there is one which is divisible by 5, and at least one which is divisible by 4, 3, and 2. That's because an integer divided by an integer $$k$$ can give a remainder of: 0 (when it's divisible by $$k$$), 1, 2, ..., or $$k-1$$ (total of $$k$$ different remainders from 0 to $$k-1$$), so out of $$k$$ consecutive integers there definitely will be one which gives a reminder of zero, so divisible by $$k$$.

Which give us the following property: the product of $$k$$ consecutive integers is always divisible by $$k!$$, so by $$k$$ too. For example: given $$k=4$$ consecutive integers $$\{3,4,5,6\}$$ --> the product of 3*4*5*6 is 360, which is divisible by 4!=24.

If we apply this property to the original question we'll have that the product of given 3 consecutive integers $$(n+4)(n+5)(n+6)$$ must be divisible by 3!=6.

Hope it's clear.
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08 Mar 2011, 19:14
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Product of Any three consecutive integers must be divisible by 6.
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Re: The function f(m) is defined for all positive integers m as [#permalink]

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29 Mar 2015, 18:21
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Hi All,

This question is based on a couple of Number Properties; as such, you can also TEST VALUES to get to the solution.

We're told that the f(M) = (M+4)(M+5)(M+6) for all POSITIVE integers. We're asked which of the following numbers MUST divide into f(N).

IF....
N = 1
f(1) = (5)(6)(7)

At this point, you can either multiply out the numbers and check the 5 answer choices against that product OR prime factor the f(1)....

(5)(6)(7) = (5)(2)(3)(7)

Of the 5 answer choices, only 2 of them divide into this product (Answers B and C; 5 and 6).

From here, we should look to try to eliminate one of the options. It's actually not that hard....

IF....
N = 2
f(2) = (6)(7)(8)

Looking at this, we can see that 6 IS a factor while 5 is NOT.

[Reveal] Spoiler:
C

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Rich
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# Special Offer: Save 75 + GMAT Club Tests 60-point improvement guarantee www.empowergmat.com/ ***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*********************** Manager Joined: 07 Jan 2010 Posts: 147 Location: So. CA WE 1: 2 IT WE 2: 4 Software Analyst Followers: 2 Kudos [?]: 68 [0], given: 57 Re: Defined Functions [#permalink] ### Show Tags 27 Jan 2011, 21:45 awesome explanation Senior Manager Joined: 08 Nov 2010 Posts: 417 WE 1: Business Development Followers: 7 Kudos [?]: 106 [0], given: 161 Re: Defined Functions [#permalink] ### Show Tags 29 Jan 2011, 11:59 Ye, Bunuel - u rocked it. +1. _________________ GMAT Club Legend Joined: 09 Sep 2013 Posts: 13522 Followers: 577 Kudos [?]: 163 [0], given: 0 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 03 Oct 2013, 18:46 Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________ GMAT Club Legend Joined: 09 Sep 2013 Posts: 13522 Followers: 577 Kudos [?]: 163 [0], given: 0 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 13 Nov 2014, 23:34 Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________ Intern Joined: 22 Nov 2012 Posts: 22 Location: United States Followers: 0 Kudos [?]: 7 [0], given: 90 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 29 Mar 2015, 10:19 Bunuel Very informative explanation Bunuel! Thanks ! _________________ GMAT, It is not finished untill I win!!! GMAT Club Legend Joined: 09 Sep 2013 Posts: 13522 Followers: 577 Kudos [?]: 163 [0], given: 0 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 14 Apr 2016, 22:24 Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________ Intern Joined: 31 May 2015 Posts: 1 Concentration: Marketing, Finance Schools: Wharton '17 GMAT 1: Q V Followers: 0 Kudos [?]: 0 [0], given: 40 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 30 Sep 2016, 21:33 [align=]I tried the sum with f(13), which is equal to 17 X 18 X 19 (this number isn't divisible by 6)?[/align] _________________ PNH0505 EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 8336 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: 340 Q170 V170 Followers: 382 Kudos [?]: 2472 [0], given: 163 Re: The function f(m) is defined for all positive integers m as [#permalink] ### Show Tags 01 Oct 2016, 15:45 Hi pnh0505, You are correct that when you use M=13, then f(13) = (17)(18)(19). However, that product IS divisible by 6 - because 18 is divisible by 6. GMAT assassins aren't born, they're made, Rich _________________ # Rich Cohen Co-Founder & GMAT Assassin # Special Offer: Save75 + GMAT Club Tests

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Re: The function f(m) is defined for all positive integers m as   [#permalink] 01 Oct 2016, 15:45
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