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# For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d

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Director
Status: Finally Done. Admitted in Kellogg for 2015 intake
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For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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24 Jan 2012, 17:12
3
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21
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Difficulty:

55% (hard)

Question Stats:

64% (02:31) correct 36% (01:58) wrong based on 414 sessions

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For any four digit number, abcd, *abcd*= (3^a)(5^b)(7^c)(11^d). What is the value of (n – m) if m and n are four digit numbers for which *m* = (3^r)(5^s)(7^t)(11^u) and *n* = (25)(*m*)?

A. 2000
B. 200
C. 25
D. 20
E. 2

Guys - any idea how to solve this please? I am struggling and OA is not given either.
[Reveal] Spoiler: OA

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Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610
GMAT ==> 730

Last edited by Bunuel on 13 Feb 2012, 04:28, edited 2 times in total.
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Re: For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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25 Jan 2012, 04:28
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Expert's post
enigma123 wrote:
For any four digit number, abcd, *abcd*= (3^a)(5^b)(7^c)(11^d). What is the value of (n – m) if m and n are four digit numbers for which *m* = (3^r)(5^s)(7^t)(11^u) and *n* = (25)(*m*)?

A. 2000
B. 200
C. 25
D. 20
E. 2

Guys - any idea how to solve this please? I am struggling and OA is not given either.

Also, if you find to difficult to grasp a question with many variables, try throwing in some values. It helps you handle the question.

abcd is a four digit number where a, b, c and d are the 4 digits.
*abcd*= (3^a)(5^b)(7^c)(11^d). The '**' act as an operator.

Given: *m* = (3^r)(5^s)(7^t)(11^u)
So m = rstu where r, s, t, and u are the 4 digits of m.
Say, r = 1 and s = 0, t = 0 and u = 0
m = 1000
Then *m* = 3

Now,
*n* = (25)(*m*) = 25(3) = (3^1)(5^2)(7^0)(11^0)
n = 1200

n - m = 1200 - 1000 = 200
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Kudos [?]: 106522 [4] , given: 11627

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24 Jan 2012, 17:27
4
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Expert's post
14
This post was
BOOKMARKED
enigma123 wrote:
For any four digit number, abcd, *abcd*= (3^a)(5^b)(7^c)(11^d). What is the value of (n – m) if m and n are four digit numbers for which *m* = (3^r)(5^s)(7^t)(11^u) and *n* = (25)(*m*)?

A. 2000
B. 200
C. 25
D. 20
E. 2

Guys - any idea how to solve this please? I am struggling and OA is not given either.

Given for four digit number, $$abcd$$, $$*abcd*=3^a*5^b*7^c*11^d$$;

From above as $$*m*=3^r*5^s*7^t*11^u$$ then four digits of $$m$$ are $$rstu$$;

Next, $$*n*=25*\{*m*\}=5^2*(3^r*5^s*7^t*11^u)=3^r*5^{(s+2)}*7^t*11^u$$, hence four digits of $$n$$ are $$r(s+2)tu$$, note that $$s+2$$ is hundreds digit of $$n$$;

You can notice that $$n$$ has 2 more hundreds digits and other digits are the same, so $$n$$ is 2 hundreds more than $$m$$: $$n-m=200$$.

Or represent four digits integer $$rstu$$ as $$1000r+100s+10t+u$$ and four digit integer $$r(s+2)tu$$ as $$1000r+100(s+2)+10t+u$$ --> $$n-m=(1000r+100(s+2)+10t+u)-1000r+100s+10t+u=200$$.

_________________
Director
Status: Finally Done. Admitted in Kellogg for 2015 intake
Joined: 25 Jun 2011
Posts: 537
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WE: Information Technology (Consulting)
Followers: 77

Kudos [?]: 3382 [0], given: 217

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24 Jan 2012, 17:31
Sorry buddy - apologies if I am missing something. But how did you get

four digits of m as r-s-t-u;
_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610
GMAT ==> 730

Math Expert
Joined: 02 Sep 2009
Posts: 39049
Followers: 7753

Kudos [?]: 106522 [0], given: 11627

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24 Jan 2012, 17:37
enigma123 wrote:
Sorry buddy - apologies if I am missing something. But how did you get

four digits of m as r-s-t-u;

For four digit integer $$abcd$$ some function, denoted by **, defined as $$*abcd*=3^a*5^b*7^c*11^d$$.

Now, as given that $$*m*=3^r*5^s*7^t*11^u$$ then four digits of m are $$rstu$$, the same way as above: $$*rstu*=3^r*5^s*7^t*11^u$$

Hope it's clear.
_________________
Director
Status: Finally Done. Admitted in Kellogg for 2015 intake
Joined: 25 Jun 2011
Posts: 537
Location: United Kingdom
GMAT 1: 730 Q49 V45
GPA: 2.9
WE: Information Technology (Consulting)
Followers: 77

Kudos [?]: 3382 [0], given: 217

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24 Jan 2012, 17:42
Ahhh - so four digits are r, s, t and u. And not r minus s minus t minus u. That's where I got confused.
_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610
GMAT ==> 730

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Posts: 15511
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Kudos [?]: 210 [0], given: 0

Re: For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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05 Mar 2014, 10:12
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Re: For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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05 Apr 2015, 22:18
Hello from the GMAT Club BumpBot!

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For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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14 Dec 2015, 08:33
Hi Bunuel

Can we arrive at the solution by the following approach ?

Given: *m* = 3^r*5^s*7^t*11^u
*n* = 25 (*m*)

To Solve: n - m

Sol: Substituting for n ,
n - m = 25 *m* - *m*
= *m* (25-1)
= *m* (24)
we know that, 24 = 3*2^3 and *m* = 3^r*5^s*7^t*11^u , does not have 2 value which implies the answer should have 2^3 as a factor.

1. 2000 = 5^3*2^4 - ( Only 2^3 is possible. as 24 has only 2^3 and *m* is not a factor of 2)
2. 200 = 5^2*2^3 - Correct
3. 25 = 5^2
4. 20 = 5 *2^2
5. 2 = 2
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Re: For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d [#permalink]

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13 Mar 2017, 12:06
Hello from the GMAT Club BumpBot!

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Re: For any four digit number, abcd, *abcd*=3^a*5^b*7^c*11^d   [#permalink] 13 Mar 2017, 12:06
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