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Senior Manager  Joined: 30 Oct 2004
Posts: 269
If a and b are positive even integers, and the least common  [#permalink]

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18 00:00

Difficulty:   65% (hard)

Question Stats: 59% (02:00) correct 41% (01:48) wrong based on 531 sessions

### HideShow timer Statistics If a and b are positive even integers, and the least common multiple of a and b is expressed as a*b/n, which of the following statements could be false?

A. n is a factor of both a and b
B. (a*b)/n < ab
C. ab is multiple of 2.
D. (a*b)/n is a multiple of 2.
E. n is a multiple of 4.

Originally posted by vikramm on 11 Oct 2005, 18:32.
Last edited by Bunuel on 28 Aug 2013, 04:21, edited 1 time in total.
Renamed the topic and edited the question.
Director  Joined: 14 Dec 2012
Posts: 736
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Concentration: General Management, Operations
GMAT 1: 700 Q50 V34 GPA: 3.6
Re: This one from Kaplan.... If 'a' and 'b' are positive even  [#permalink]

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vikramm wrote:
This one from Kaplan....

If 'a' and 'b' are positive even integers, and the least common multiple of 'a' and 'b' is expressed as ab/n, which of the following could be false?
A) n is a factor of both a and b
B) ab/n < ab
C) ab is a multiple of 2
D) ab/n is a multiple of 2
E) n is a multiple of 4

This question is easier if you know that, for any positive integers a and b:

LCM(a,b) * GCD(a,b) = a*b

So LCM(a,b) = ab/GCD(a,b)

In this question, n is just the GCD of a and b. So we know automatically that n is a factor of a and b... A) must be true.
If a and b are both even, they are both divisible by 2, so the smallest possible value for their GCD is 2. That is, n >= 2, so B) must be true
If a and b are even, ab is clearly even, and so is the LCM of a and b, so C) and D) must be true.
E) is the only one that might be false; E will only be true if both a and b are divisible by 4.

hence E
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Director  Joined: 21 Aug 2005
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E may be false.
If a=4, b=2 least comm. multiple is 4.
ab/n=4 => n=2 which is not a multiple of 4
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Re: This one from Kaplan.... If 'a' and 'b' are positive even  [#permalink]

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blueseas wrote:
vikramm wrote:
This one from Kaplan....

If 'a' and 'b' are positive even integers, and the least common multiple of 'a' and 'b' is expressed as ab/n, which of the following could be false?
A) n is a factor of both a and b
B) ab/n < ab
C) ab is a multiple of 2
D) ab/n is a multiple of 2
E) n is a multiple of 4

E) is the only one that might be false; E will only be true if both a and b are divisible by 4.

hence E

Any information is provided such that a and b cannot be a multiple of 4 ( 'a' and 'b' are positive even integers - a can be 4 and b can be 16 or 8) ? I don't think it is valid question.
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: If a and b are positive even integers, and the least common  [#permalink]

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2
Hi All,

The last poster in this thread (kinjiGC) had some concerns about whether this question was "valid" or not, but I can assure you that it is.

The question is based on Number Property rules that exist in the realm of factors and multiples. Given the restrictions in the prompt, 4 of the answers are ALWAYS TRUE....

For example, since we're told that A and B are POSITIVE EVEN INTEGERS, we know that (A)(B) = (even)(even), so the product will ALWAYS be EVEN. Thus AB is a multiple of 2 and we can eliminate Answer C (since we're asked to find something that is SOMETIMES FALSE - meaning NOT ALWAYS TRUE).

These types of "...must be true..." questions almost always involve Number Properties of some kind, so you can TEST VALUES or use Number Property Rules to get to the correct answer. You'll see a few questions in this general "format" on Test Day and a bunch of questions involving Number Properties in general (especially in DS).

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Manager  Joined: 01 May 2013
Posts: 61
Re: If a and b are positive even integers, and the least common  [#permalink]

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If a and b are positive even integers, and the least common multiple of a and b is expressed as a*b/n, which of the following statements could be false?

A. n is a factor of both a and b
B. (a*b)/n < ab
C. ab is multiple of 2.
D. (a*b)/n is a multiple of 2.
E. n is a multiple of 4.

If we divide by n and get a multiple of a and of b, then n must be a common factor. Answer A must be true.

If a and b are even integers, dividing by a factor (also an integer) results in something smaller than the product ab. Answer B must be true.

If either a or b is even, then ab must be even. Answer C must be true.

If both are even, the smallest either number can be is 2. Thus, the smallest common factor must be 2. Answer D must be true.

If b is 6 and a is 2, n would be 6, which is not 4. E can be false.
Intern  Joined: 01 Feb 2016
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Re: If a and b are positive even integers, and the least common  [#permalink]

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1
Using a=2, b=4. LCM (2,4) = 4. This means ab/n can be written as 2*4/2 (because LCM(=4) the least common multiple of 'a' and 'b' is expressed as ab/n, 2*4/2=4=LCM). Thus n=2 (and a=2, b=4)
A. n is a factor of both a and b - True. 2 is factor for 2 and 4
B. (a*b)/n < ab - True (4<8)
C. ab is multiple of 2. - True (8 is multiple of 2)
D. (a*b)/n is a multiple of 2. - True (4 is multiple of 2)
E. n is a multiple of 4. False (n=2 is not multiple of 4)
Manager  Joined: 15 Apr 2016
Posts: 68
Re: If a and b are positive even integers, and the least common  [#permalink]

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1
Using a=2, b=4. LCM (2,4) = 4. This means ab/n can be written as 2*4/2 (because LCM(=4) the least common multiple of 'a' and 'b' is expressed as ab/n, 2*4/2=4=LCM). Thus n=2 (and a=2, b=4)
A. n is a factor of both a and b - True. 2 is factor for 2 and 4
B. (a*b)/n < ab - True (4<8)
C. ab is multiple of 2. - True (8 is multiple of 2)
D. (a*b)/n is a multiple of 2. - True (4 is multiple of 2)
E. n is a multiple of 4. False (n=2 is not multiple of 4)

Great Explanation ! Thanks !
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Re: If a and b are positive even integers, and the least common  [#permalink]

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_________________ Re: If a and b are positive even integers, and the least common   [#permalink] 15 Jul 2018, 02:52
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