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A positive integer N has exactly 4 unique positive factors. What is th

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A positive integer N has exactly 4 unique positive factors. What is th  [#permalink]

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New post 21 Mar 2019, 22:21
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A positive integer N has exactly 4 unique positive factors. What is the highest prime factor of N?

(1) 5 is a factor of N.
(2) 25 is a factor of N.
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A positive integer N has exactly 4 unique positive factors. What is th  [#permalink]

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New post 30 Mar 2019, 00:51
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sumi747 wrote:
I am still not clear with the explanation as to how p would be only 5 and nothing greater than it. Could you try elaborating?

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Hey sumi747,

Because if you had a different prime factor, there would be more than 4 positive factors total.
For example, say 7 was a factor.
Since 25 is a factor, then 7*25=175 is also a factor, and so are 5*7=35 and 7.
So N must have 1,5,25,7,35,175 as factors, meaning it has more than 4 unique factors, a contradiction.
In general, for any prime p other than 5, you will have 1,5,25,p,5p,25p so at least 6 factors. Then the only prime factor of N is 5.
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New post Updated on: 30 Mar 2019, 00:52
amanvermagmat wrote:
A positive integer N has exactly 4 unique positive factors. What is the highest prime factor of N?

(1) 5 is a factor of N.
(2) 25 is a factor of N.


As we have very few factors we have very few options so we can just write them out.
This is an Alternative approach.

(1) Then (1,5) are both factors of N. If prime p other than 5 is also a factor, then the 4 factors must be (1,5,p,5p). If p > 5, then the answer to the quesiton is 'p', but as there are many options for p, this is insufficient.

(2) Then (1,5,25) are factors of N. Similarly to the above, choosing a prime p other than 5 gives (1,5,25,p,5p,25p), too much! So the only prime factor of N is 5 and it is (trivially) also the highest prime factor.
Sufficient.

(B) is our answer
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Originally posted by DavidTutorexamPAL on 29 Mar 2019, 08:34.
Last edited by DavidTutorexamPAL on 30 Mar 2019, 00:52, edited 1 time in total.
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Re: A positive integer N has exactly 4 unique positive factors. What is th  [#permalink]

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New post 30 Mar 2019, 00:45
DavidTutorexamPAL wrote:
amanvermagmat wrote:
A positive integer N has exactly 4 unique positive factors. What is the highest prime factor of N?

(1) 5 is a factor of N.
(2) 25 is a factor of N.


As we have very few factors we have very few options so we can just write them out.
This is an Alternative approach.

(1) Then (1,5) are both factors of N. If prime p other than 5 is also a factor, then the 4 factors must be (1,5,p,5p). If p > 5, then the answer to the quesiton is 'p', but as there are many options for p, this is insufficient.

(2) Then (1,5,25) are factors of N. Similarly to the above, choosing a prime p other than 5 gives (1,5,25,p,5p,25p), too much! So the only prime factor of p is 5 and it is (trivially) also the highest prime factor.
Sufficient.

(B) is our answer

DavidTutorexamPAL I am still not clear with the explanation as to how p would be only 5 and nothing greater than it. Could you try elaborating?

Posted from my mobile device
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A positive integer N has exactly 4 unique positive factors. What is th  [#permalink]

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New post 31 Mar 2019, 16:41
St.1:
2x5= 10, highest PF= 5
5x7= 35, highest PF= 7
And so on
Insufficient.

Caution: Do not carry out thoughts from Statement 1 to Statement 2.

St.2:
5^2 is a factor of N.
And total factors of N is 4.

In this case, if another PF besides 5 is a part of N, then min total factors of N will become (2+1)*(1+1)= 3*2= 6.

So, for N to have 4 factors, value of N should be 5^3.
Highest PF is 5.
Sufficient

Ans B
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A positive integer N has exactly 4 unique positive factors. What is th   [#permalink] 31 Mar 2019, 16:41
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