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GRE math Challenge #43-If N = 3^x*5^y, where x and y

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GRE math Challenge #43-If N = 3^x*5^y, where x and y [#permalink]  28 Feb 2015, 03:27
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If $$N = 3^x*5^y$$, where x and y are positive integers, and N has 12 positive factors, what is the value of N?

 Column A Column B 9 is not a factor 125 is a factor

A. Only Column A is sufficient.
B. Only Column B is sufficient.
C. Column A and B both are needed
D. Column A and B both are insufficient
E. None of the columns are needed.
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Re: GRE math Challenge #43 [#permalink]  26 Mar 2015, 16:31
That is not a GRE question, just FYI.

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Re: GRE math Challenge #43 [#permalink]  31 Mar 2015, 09:10
Expert's post
Yes you are right!
The idea is to solve maths problems regardless what exam it is. Just to challenge brains.
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Re: GRE math Challenge #43-If N = 3^x*5^y, where x and y [#permalink]  11 Mar 2018, 05:38
soumya1989 wrote:
If $$N = 3^x*5^y$$, where x and y are positive integers, and N has 12 positive factors, what is the value of N?

 Column A Column B 9 is not a factor 125 is a factor

A. Only Column A is sufficient.
B. Only Column B is sufficient.
C. Column A and B both are needed
D. Column A and B both are insufficient
E. None of the columns are needed.

For me, the ans would be A, we are told that $$N = 3^x*5^y$$, where x and y are positive integers, and N has 12 positive factors.

then (x+1)(y+1)=12. Let us do some prime factorization of 12.

12=2*6=1*12=3*4 We should rule out the pair 1*12 since x+1 or y+1=1, then x or y=0. Yet, we are told that x and y are positive integers. Now for cloumn A, we are told that 9 is not a factor of N. This leads us to know x=1 and y=5. then $$N = 3^1*5^5$$.

For column B, we are told that 125 is a factor. That means that y>=3. So, y would be 3 or 5. B can't help us decide the value of N.

ANS:A
Re: GRE math Challenge #43-If N = 3^x*5^y, where x and y   [#permalink] 11 Mar 2018, 05:38
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