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If a and b are positive even integers, and the least common [#permalink]

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11 Oct 2005, 18:32

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55% (01:30) correct
45% (06:54) wrong based on 485 sessions

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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 '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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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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Re: If a and b are positive even integers, and the least common [#permalink]

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23 Jan 2015, 13:22

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).

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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).

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

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24 Jan 2015, 13:01

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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.

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

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08 Jun 2016, 21:45

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).

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

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15 Jun 2016, 00:03

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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)

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

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31 Jul 2016, 05:22

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manuchadha wrote:

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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