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# If n and t are positive integers, what is the greatest prime

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If n and t are positive integers, what is the greatest prime [#permalink]

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29 Jan 2012, 17:24
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If n and t are positive integers, what is the greatest prime factor of nt?

(1) The greatest common factor of n and t is 5
(2) The least common multiple of n and t is 105

[Reveal] Spoiler:
Ok - I have got the correct answer B, but it was a guess work on statement 1.

Considering statement 1

n and t can have several values for example n = 25, t = 105. GCF will be 5 but the greatest Prime factor will be 7. However if n and t both equals 5 then GCF will be 5 but greatest prime factor will be 5. Therefore this statement is insufficient. Am I right or wrong?

Considering statement 2

It means that greatest value of n & t is 105 and prime factors of 105 are 1,3,5 and 7. Therefore 7 is the greatest Prime factor. Am I right?
[Reveal] Spoiler: OA

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Re: Greatest Prime Factor of nt [#permalink]

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29 Jan 2012, 17:40
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If n and t are positive integers, what is the greatest prime factor of nt?

(1) The greatest common factor of n and t is 5 --> if n and t does not have any prime greater than 5 then the greatest prime factor of nt will be 5 (example: n=5 and t=5 or n=10 and t=15) BUT if n and/or t have some primes more than 5 then the greatest prime factor of nt will be more than 5 (example: n=35 and t=5 --> the greatest prime of nt is 7 or n=5 and t=55 --> the greatest prime of nt is 11)

(2) The least common multiple of n and t is 105 --> the least common multiple of two integers contains all common primes of these integers, thus 105 has all the primes which appear in both n and t --> the greatest prime factor of 105 is 7, hence it's the the greatest prime factor of nt (no greater factor can "appear" in nt if it's not in either of them). Sufficient.

Hope it's clear.
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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14 May 2014, 04:51
Bunuel

It is quite agreed that LCM of two integers will contain all the primes of these integers, does the same hold true for GCF of two numbers as well?
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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14 May 2014, 06:50
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gaurav1418z wrote:
Bunuel

It is quite agreed that LCM of two integers will contain all the primes of these integers, does the same hold true for GCF of two numbers as well?

The greatest common divisor (gcd), also known as the greatest common factor (gcf), or highest common factor (hcf), of two or more non-zero integers, is the largest positive integer that divides the numbers without a remainder.

To find the GCF, you will need to do prime-factorization. Then, multiply the common factors (pick the lowest power of the common factors).

Hence the GCF will contain only the common primes of two number. For example, the GCF of 36=2^2*3^2 and 100=2^2*5^2 is 4=2^2.

Theory on Number Properties: math-number-theory-88376.html

All DS Number Properties Problems to practice: search.php?search_id=tag&tag_id=38
All PS Number Properties Problems to practice: search.php?search_id=tag&tag_id=59

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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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26 May 2015, 23:17
Hi All,

Prime factorization is a very strong tool while solving LCM-GCD questions. This question directly tests the process knowledge of prime factorization method of finding LCM-GCD.

For further refinement of your approach in LCM-GCD questions and avoiding fatal mistakes please go through the below article on mistakes committed by students in LCM-GCD questions.

3 Deadly Mistakes you must avoid in LCM-GCD questions

Hope this helps

Regards
Harsh
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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31 Jan 2017, 22:18
Statement 1: GCF(n,t) = 5

if n=15, t=10, greatest prime factor of nt = 5
if n=21, t=15, greatest prime factor of nt = 7. Hence insufficient.

Statement 2: LCM(n,t) = 105 = 3x5x7

7 is the greatest prime factor for 105.

either n or t or both must contain 7 as a factor. Therefore 7 is the greatest prime factor of nt. Sufficient.

Choose B.
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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16 Feb 2017, 06:13
Hi guys, I got quite confused after seeing this question in my practice test.

There are no sets of numbers for n and t in whichh the GCF and 5 AND the LCM is 105. Sentences 1 and 2 therefore appear to be inconsistent.

My question is - for data sufficiency questions, aren't the 2 sentences supposed to be consistent with each other and the initial question? Or am I missing something here?

Thanks!

Posted from my mobile device
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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16 Feb 2017, 06:30
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bigbanana14 wrote:
Hi guys, I got quite confused after seeing this question in my practice test.

There are no sets of numbers for n and t in whichh the GCF and 5 AND the LCM is 105. Sentences 1 and 2 therefore appear to be inconsistent.

My question is - for data sufficiency questions, aren't the 2 sentences supposed to be consistent with each other and the initial question? Or am I missing something here?

Thanks!

Posted from my mobile device

On the GMAT, two data sufficiency statements always provide TRUE information and these statements NEVER contradict each other or the stem.

But the statements in the original questions does not contradict. Consider
n = 5 and t = 105
n = 15 and t = 35

The greatest common factor of n and t is 5 and the least common multiple of n and t is 105.
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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19 Aug 2017, 09:25
I keep stumbling wrt statement 2 here. What if n is 11x7x5x3 and t is 7x5x3. Then the LCM is still 7x5x3=105 but the greatest prime factor of nt is 11?
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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19 Aug 2017, 09:32
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shahidkhan42 wrote:
I keep stumbling wrt statement 2 here. What if n is 11x7x5x3 and t is 7x5x3. Then the LCM is still 7x5x3=105 but the greatest prime factor of nt is 11?

You are mixing the least common multiple and the greatest common factor. 105 is the greatest common factor (GCF) of 11*7*5*3 and 7*5*3, not the least common multiple (LCM).
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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19 Aug 2017, 09:49
Bunuel wrote:
shahidkhan42 wrote:
I keep stumbling wrt statement 2 here. What if n is 11x7x5x3 and t is 7x5x3. Then the LCM is still 7x5x3=105 but the greatest prime factor of nt is 11?

You are mixing the least common multiple and the greatest common factor. 105 is the greatest common factor (GCF) of 11*7*5*3 and 7*5*3, not the least common multiple (LCM).

Got it. I knew I was messing up something. Thanks.
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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20 Aug 2017, 02:25
Bunuel wrote:
If n and t are positive integers, what is the greatest prime factor of nt?

(1) The greatest common factor of n and t is 5 --> if n and t does not have any prime greater than 5 then the greatest prime factor of nt will be 5 (example: n=5 and t=5 or n=10 and t=15) BUT if n and/or t have some primes more than 5 then the greatest prime factor of nt will be more than 5 (example: n=35 and t=5 --> the greatest prime of nt is 7 or n=5 and t=55 --> the greatest prime of nt is 11)

Dear Bunuel,

In Statement 1:

If n = 5 & t =45....then GCF =5 but what is the greatest prime factor of nt?? Is it 3^2 or 5?? When used term greatest, should we include power or ignore it?

Thanks
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Re: If n and t are positive integers, what is the greatest prime [#permalink]

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20 Aug 2017, 02:30
Mo2men wrote:
Bunuel wrote:
If n and t are positive integers, what is the greatest prime factor of nt?

(1) The greatest common factor of n and t is 5 --> if n and t does not have any prime greater than 5 then the greatest prime factor of nt will be 5 (example: n=5 and t=5 or n=10 and t=15) BUT if n and/or t have some primes more than 5 then the greatest prime factor of nt will be more than 5 (example: n=35 and t=5 --> the greatest prime of nt is 7 or n=5 and t=55 --> the greatest prime of nt is 11)

Dear Bunuel,

In Statement 1:

If n = 5 & t =45....then GCF =5 but what is the greatest prime factor of nt?? Is it 3^2 or 5?? When used term greatest, should we include power or ignore it?

Thanks

3^2 = 9 and it's not prime, so you should not consider it.

If n = 5 and t = 45, then $$nt = 5*45 = 3^2*5^2$$. So, the greatest common factor of nt is 5.
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Re: If n and t are positive integers, what is the greatest prime   [#permalink] 20 Aug 2017, 02:30
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