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Re: If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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20 Mar 2013, 16:40

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If a positive integer n,divided by 5 has a remainder 2,which of the following must be true I. n is odd II. n+1 cannot be a prime number III. (n+2)divided by 7 has remainder 2

Some valid values for n: 7, 12, 17, 22, 27, 32... or, in other words: \(n=(i * 5) + 2\) for i=1,2,3...

I. FALSE: we see that n can we odd or even. II. FALSE: (n+1) could be a prime number. Example: n=12 --> (n+1)=13 is prime. Other example: for n=22, (n+1)=23 is prime. III. FALSE: for n=12, (n+2)=14, divided by 7 has remainder zero.

Re: If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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20 Mar 2013, 21:54

chiccufrazer1 wrote:

If a positive integer n,divided by 5 has a remainder 2,which of the following must be true I. n is odd II. n+1 cannot be a prime number III. (n+2)divided by 7 has remainder 2

A.none B.I only C.I and II only D.II and III only E.I,II and III

n can be written as :

n = 5k+2. Thus, taking k=0, we have n=2.

I.n=2,even.False II.2+1=3, is a prime. False. III.n+2 = 4,4 divided by 7 leaves a remainder of 4. False.

Re: If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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21 Mar 2013, 01:36

chiccufrazer1, you forgot to provide the OA in your post. Just make sure you do provide it for your future problems.

Alright, let's solve this :

We know that n, a positive integer, yields a remainder of 2 when divided by 5. So according to the algebraic form of the division operation, we'll have :

\(n = 5*q + 2\) with q being a positive integer as well.

This expression allows us to give out some valid possibilities for n by playing with the value of q, such as :

q = 0 => n = 2 q = 1 => n = 7 q= 2 => n =12

Now, from these first values we can already cross off statement I.(n is odd), since n can be 7 (which is odd) or n can be 12 (which is even).

Statement II. (n+1 cannot be a prime number) can also be crossed off. Consider n = 12, which is not a prime number and yields a remainder of 2 when divided by 5. If we add 1 to it, we get 13, which IS a prime number, so that contradicts statement II.

Finally, statement III. (n+2 yields a remainder of 2 when divided by 7) can also be crossed off. Again consider n = 12. Add 2 to it and we get a 14 which is a multiple of 7.

In short, all statements have been contradicted and the correct answer choice to the question is A : none of the statements above are true.

Re: If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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13 Nov 2014, 10:49

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Re: If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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05 Jul 2017, 15:53

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If a positive integer n, divided by 5 has a remainder 2 [#permalink]

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06 Jul 2017, 09:11

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Bunuel wrote:

If a positive integer n, divided by 5 has a remainder 2, which of the following must be true

I. n is odd II. n+1 cannot be a prime number III. (n+2) divided by 7 has remainder 2

A. None B. I only C. I and II only D. II and III only E. I, II and III

Quote:

A positive integer n, divided by 5 has a remainder 2 --> \(n=5q+2\), so n could be 2, 7, 12, 17, 22, 27, ...

I. n is odd. Not necessarily true, since n could be 2, so even.

II. n+1 cannot be a prime number. Not necessarily true, since n could be 2, so n+1=3=prime.

III. (n+2) divided by 7 has remainder 2. Not necessarily true, since n could be 7, so n+2=9.

Answer: A.

Hope it's clear.

Bunuel , I am confused by your analysis of III: (n+2) divided by 7 has remainder 2

If n = 7 and (n + 2) = 9, then \(\frac{9}{7}\) = 1 + R2.

n could be 2, 7, 12, 17 ...

If n = 12, then (n+2) = 14, which, when divided by 7, leaves remainder 0.

If n = 17, (n+2) = 19, which, when divided by 7, leaves remainder 5.

Those two examples (or others) seem to me to be what should be used to show that III does not satisfy the condition "must be true."

The one you chose proves that III could be true; I'm having a hard time understanding how n = 7 proves that III does not have to be true. Am I missing something?

Last edited by genxer123 on 06 Jul 2017, 09:24, edited 1 time in total.

If a positive integer n, divided by 5 has a remainder 2, which of the following must be true

I. n is odd II. n+1 cannot be a prime number III. (n+2) divided by 7 has remainder 2

A. None B. I only C. I and II only D. II and III only E. I, II and III

Quote:

A positive integer n, divided by 5 has a remainder 2 --> \(n=5q+2\), so n could be 2, 7, 12, 17, 22, 27, ...

I. n is odd. Not necessarily true, since n could be 2, so even.

II. n+1 cannot be a prime number. Not necessarily true, since n could be 2, so n+1=3=prime.

III. (n+2) divided by 7 has remainder 2. Not necessarily true, since n could be 7, so n+2=9.

Answer: A.

Hope it's clear.

Bunuel , I am confused by your analysis of III: (n+2) divided by 7 has remainder 2

If n = 9, then \(\frac{9}{7}\) = 1 + R2.

n could be 2, 7, 12, 17 ...

If n = 12, then (n+2) = 14, which, when divided by 7, leaves remainder 0.

If n = 17, (n+2) = 19, which, when divided by 7, leaves remainder 5.

Those two examples (or others) seem to me to be what should be used to show that III does not satisfy the condition "must be true."

The one you chose proves that III could be true; I'm having a hard time understanding how n = 7 proves that III does not have to be true. Am I missing something?

You are right. Edited the question.
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