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# If n = x^5*y^7, where x and y are positive integers greater than 1

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Manager
Joined: 06 Oct 2015
Posts: 91
Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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08 Aug 2016, 06:33
RK84 wrote:
Sorry it's a typo ..the correct answer has to be C (100%)

Sent from my iPhone using GMAT Club Forum mobile app

Can you explain in detail?
Manager
Joined: 06 Oct 2015
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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08 Aug 2016, 06:38
MarkusKarl wrote:
I'll give it a try.

A)
X, Y = prime.

If X =/= Y then we would know the answer (6*8 factors). However, if X=Y then we would receive a lot of answers that would generate the same factors with the previous method. e.g. x=y=3 => 3^3*3^1=3^1*3^3.

Thus, A is insufficient.

As for B) (Which, in all honesty, was my first choice before I analyzed the question further).
We now know that x and y consist of 2 prime numbers.

IF x and y are two different prime numbers we would be able to answer the question. However(and here was my mistake), that information is given in (A). (In that case it would indeed be 6*8 factors).

X and Y can still be different composite numbers without giving n more than 2 prime factors. e.g. x = 2*3 and y = 2*2*3.
X and Y can also be composite numbers where x and y are two different prime numbers taken to any power.

Thus, (as written above) We needed the information about x and y being prime numbers from (A) and we needed the information that n only had two different prime factors from (B). Thus -> C is the answer.

Great question, thanks!

Can a single prime number work for both X and Y? If so then it should be written X and X, I think, not X and Y.
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Joined: 15 Mar 2015
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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08 Aug 2016, 07:21
NaeemHasan wrote:
MarkusKarl wrote:
I'll give it a try.

A)
X, Y = prime.

If X =/= Y then we would know the answer (6*8 factors). However, if X=Y then we would receive a lot of answers that would generate the same factors with the previous method. e.g. x=y=3 => 3^3*3^1=3^1*3^3.

Thus, A is insufficient.

As for B) (Which, in all honesty, was my first choice before I analyzed the question further).
We now know that x and y consist of 2 prime numbers.

IF x and y are two different prime numbers we would be able to answer the question. However(and here was my mistake), that information is given in (A). (In that case it would indeed be 6*8 factors).

X and Y can still be different composite numbers without giving n more than 2 prime factors. e.g. x = 2*3 and y = 2*2*3.
X and Y can also be composite numbers where x and y are two different prime numbers taken to any power.

Thus, (as written above) We needed the information about x and y being prime numbers from (A) and we needed the information that n only had two different prime factors from (B). Thus -> C is the answer.

Great question, thanks!

Can a single prime number work for both X and Y? If so then it should be written X and X, I think, not X and Y.

Hi,

I am not sure that I understand your question, but i will try to respond.

The question does not state that x and y are different numbers. As such, we are not able to answer the question with just a. We receive that information when a and b are combined.

Please let me know if I misunderstood the question or if I can clarify further.

Best wishes

Posted from my mobile device
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Joined: 05 Aug 2016
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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08 Aug 2016, 09:52
1
If x and y are positive integers and n=x5∗y7n=x5∗y7, then how many positive divisors does n have?

(1) x does not have a factor p such that 1 < p < x and y does not have a factor q such that 1 < q < y.
(2) n has only two prime factors.

Giving it a try (This was actually my second attempt):

S1 states that X and Y are prime numbers with no current constraints

Can we find the number of positive divisors of N from that? (MGMAT says to work to prove the statement insuff)

Scen 1: (X = 2, Y = 3)
(2^5)(3^7) No of divisors (5+1)(7+1) = 48

Scen 2 (X = 2, Y = 2)
(2^12) No. of Divisors =/= 48

S1 is insufficient due to multiple answers

S2 states that N has two different prime numbers meaning that X and/or Y can be any numbers that share at most two prime factors (eg: [2,3],[2,6], [2,10], [5,15], [15,15])

Scen 1: (X = 2, Y = 3)
(2^5)(3^7) No of divisors (5+1)(7+1) = 48

Scen 2 (X = 2, Y = 6) (PF = 2 & 3)
Honestly, I don't even know what the number of divisors would be but I know it's not 48, and I'm not going to waste time trying to figure out what that number is. Knowing it's not 48 is enough for me to say it's insufficient.

So A, D, & B are eliminated

Now to try S1 & S2 together:

S1: X and Y are prime numbers & S2: X and Y must be different prime numbers

Scen 1: (X = 2, Y = 3)
(2^5)(3^7) No of divisors (5+1)(7+1) = 48

Scen 1: (X = 2, Y = 5)
(2^5)(5^7) No of divisors (5+1)(7+1) = 48

During my first try while I was scribbling on paper I read S2 as having the parameters of S1 & S2 combined. I essentially read it as X and Y have to be different prime numbers, which was the incorrect way to read it.
Manager
Joined: 06 Oct 2015
Posts: 91
Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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09 Aug 2016, 02:35
MarkusKarl wrote:
NaeemHasan wrote:
MarkusKarl wrote:
I'll give it a try.

A)
X, Y = prime.

If X =/= Y then we would know the answer (6*8 factors). However, if X=Y then we would receive a lot of answers that would generate the same factors with the previous method. e.g. x=y=3 => 3^3*3^1=3^1*3^3.

Thus, A is insufficient.

As for B) (Which, in all honesty, was my first choice before I analyzed the question further).
We now know that x and y consist of 2 prime numbers.

IF x and y are two different prime numbers we would be able to answer the question. However(and here was my mistake), that information is given in (A). (In that case it would indeed be 6*8 factors).

X and Y can still be different composite numbers without giving n more than 2 prime factors. e.g. x = 2*3 and y = 2*2*3.
X and Y can also be composite numbers where x and y are two different prime numbers taken to any power.

Thus, (as written above) We needed the information about x and y being prime numbers from (A) and we needed the information that n only had two different prime factors from (B). Thus -> C is the answer.

Great question, thanks!

Can a single prime number work for both X and Y? If so then it should be written X and X, I think, not X and Y.

Hi,

I am not sure that I understand your question, but i will try to respond.

The question does not state that x and y are different numbers. As such, we are not able to answer the question with just a. We receive that information when a and b are combined.

Please let me know if I misunderstood the question or if I can clarify further.

Best wishes

Posted from my mobile device

My point is that X and Y can not represent a sigle value. As X and Y are different letters they should reflect different values and hence A should be the answer.
Hope, you got my point now.
Intern
Joined: 05 Aug 2016
Posts: 5
Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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09 Aug 2016, 08:10
NaeemHasan wrote:
My point is that X and Y can not represent a sigle value. As X and Y are different letters they should reflect different values and hence A should be the answer.
Hope, you got my point now.

X and Y are different variables that represent some integer value, and without specific constraints they each can represent any value on the number line: -∞ < X < ∞ & -∞ < Y < ∞. This also means that, unless otherwise stated, X = Y is a possibility.
Math Expert
Joined: 02 Sep 2009
Posts: 50728
Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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09 Aug 2016, 08:52
1
NaeemHasan wrote:

My point is that X and Y can not represent a sigle value. As X and Y are different letters they should reflect different values and hence A should be the answer.
Hope, you got my point now.

Unless it is explicitly stated otherwise, different variables CAN represent the same number.
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Posts: 91
Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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09 Aug 2016, 08:56
Bunuel wrote:
NaeemHasan wrote:

My point is that X and Y can not represent a sigle value. As X and Y are different letters they should reflect different values and hence A should be the answer.
Hope, you got my point now.

Unless it is explicitly stated otherwise, different variables CAN represent the same number.

Thank you for your explanation. I didn't know it.
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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21 Aug 2016, 03:47
NaeemHasan wrote:
Statement 1 is sufficient alone. If we know that X and Y are primes then obviously we can determine the number of factors of n. Total factors are (5+1)*(7+1)=48.
But unfortunately the OA is C.
Where is my mistake? Can you, bunuel, help me to understand the fact, please.

Hi there
See if x=y => number of factors = 13
if not then the number of factors = 48
statement 2 is insufficient as x=2*3 and y=1 is different from x=3 and y=2
combining them we can say that x and y are both different primes hence the number of factors = 48
Smash that C
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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21 Nov 2016, 07:34
Hi Abhishek007
I am stuck on this one

I seriously think the answer here should be A.

Can you explain this to me.
Why is A not correct here.?

Here is my logic
Here N= x^5*y^7
Statement 1=>
x and y are prime
So number of factors of N must be 6*8=> 48
Sufficient

Statement 2
Here we can make test cases to see that its not sufficient
e.g=>
y=1
x=6
factors =36
x=7
y=5
factors = 48
Hence Insufficient

Regards
Stone Cold
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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21 Nov 2016, 07:36
Senior Manager
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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26 Mar 2017, 03:37
Bunuel wrote:
If x and y are positive integers and $$n = x^5*y^7$$, then how many positive divisors does n have?

(1) x does not have a factor p such that 1 < p < x and y does not have a factor q such that 1 < q < y.
(2) n has only two prime factors.

Bunuel

Hi..I got the correct answer..but I have a question.

Consider statement

(1) x does not have a factor p such that 1 < p < x and y does not have a factor q such that 1 < q < y.

Does this statement stop any of x and y from being = 1 ??

If we translate it in mathematical terms..it will become 1<p<1. So x = 1 is possible because nothing satisfies the translated condition..maybe I'm just being silly but it still is intriguing to me
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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26 Mar 2017, 03:51
ShashankDave wrote:
Bunuel wrote:
If x and y are positive integers and $$n = x^5*y^7$$, then how many positive divisors does n have?

(1) x does not have a factor p such that 1 < p < x and y does not have a factor q such that 1 < q < y.
(2) n has only two prime factors.

Bunuel

Hi..I got the correct answer..but I have a question.

Consider statement

(1) x does not have a factor p such that 1 < p < x and y does not have a factor q such that 1 < q < y.

Does this statement stop any of x and y from being = 1 ??

If we translate it in mathematical terms..it will become 1<p<1. So x = 1 is possible because nothing satisfies the translated condition..maybe I'm just being silly but it still is intriguing to me

1 < p < x means that x is greater than 1.
1 < q < y means that y is greater than 1.
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Re: If n = x^5*y^7, where x and y are positive integers greater than 1  [#permalink]

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16 Nov 2017, 12:03
Trap for this question is x can be equal to y.

Re: If n = x^5*y^7, where x and y are positive integers greater than 1 &nbs [#permalink] 16 Nov 2017, 12:03

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