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Positive interger n

This topic has 3 member replies
samba7 Newbie | Next Rank: 10 Posts Default Avatar
Joined
20 Nov 2006
Posted:
4 messages

Positive interger n

Post Sun Dec 03, 2006 2:06 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Question

    Can the positive interger n be written as the sum of two different positive
    prime numbers?

    1. n is greater than 3

    2. n is odd

    when n= 5 (2+3)

    but the answer is E

    ???

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    Post Sun Dec 03, 2006 9:11 pm
    The fastest way to do these types of problems is to come up with an example that answers the question as yes and another example that gives an answer of a no.

    Statement 1:

    Supporting Example: n = 5 = 2 + 3 (Yes)
    Contradicting Example: n = 6, the only possibilities are (1, 5), (2, 4),
    (3, 3) and none of them satisfy the conditions laid out in the main statement. The case of (3, 3) can be ruled out because the two prime numbers have to be distinct.

    Statement 2:

    Supporting Example: n = 5 = 2 + 3 (Yes)
    Contradicting Example: Numbers that are odd cannot be expressed as sum of prime numbers that are both odd (because odd + odd = even), therefore, to express numbers that are odd one of the prime numbers has to be even.

    Consider the case of n = 11, the possible pair of numbers that add up to 11 are: (1, 10), (2, 9), (3, eight), (4, 7), (5, 6). None of these are made up of prime numbers alone.

    Statement 1 and 2:

    n is odd and n>3

    Use the examples of Statement 2: n = 5 (Yes) and n=11(No).

    Therefore, the answer is E.

    I hope my explanation is clear.

    Cheers,
    Mark

    samba7 Newbie | Next Rank: 10 Posts Default Avatar
    Joined
    20 Nov 2006
    Posted:
    4 messages
    Post Mon Dec 04, 2006 7:37 am
    Yes, that makes perfect sense, I was looking for a specific answer when I should have been looking for a yes/no

    Much apprecciated,

    Sam

    800guy Master | Next Rank: 500 Posts Default Avatar
    Joined
    27 Jun 2006
    Posted:
    354 messages
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    5 members
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    11 times
    Post Mon Dec 04, 2006 2:40 pm
    wow...awesome, mark. that was really clear

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