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If positive integers a and b are both odd,

This topic has 3 expert replies and 1 member reply

If positive integers a and b are both odd,

Post Mon Sep 25, 2017 6:57 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    If positive integers a and b are both odd, which of the following must not be odd?

    A) ab
    B) a(ab)
    C) b - a - 1
    D) (a + b)/2
    E) a - b

    The OA is E.

    If we set a=3 and b=5 the (a+b)/2 is 4 and it is not odd. Why option D is wrong?

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    Post Tue Sep 26, 2017 4:58 am
    Vincen wrote:
    If positive integers a and b are both odd, which of the following must not be odd?

    A) ab
    B) a(ab)
    C) b - a - 1
    D) (a + b)/2
    E) a - b

    The OA is E.

    If we set a=3 and b=5 the (a+b)/2 is 4 and it is not odd. Why option D is wrong?
    Given a and b are both odd. Just put a = 1 and b = 1, then check all the options:
    (A) ab = 1*1 = 1 Odd
    (B) a*(ab) = 1*(1*1) = 1 Odd
    (C) b - a - 1 = 1 - 1 - 1 = -1 Odd
    (D) (a+b)/2 = (1+1)/2 = 1 Odd
    (E) a - b = 1 - 1 = 0 Even

    So, the answer is E

    GMAT/MBA Expert

    Post Tue Sep 26, 2017 8:33 am
    The most important thing about this question is that it asks for the answer that must not be odd. This means that an answer will only be right if it is always odd, no matter what odd numbers we plug in for a and b. An answer will be wrong if it is 1) always even OR 2) sometimes odd, sometimes even.

    So you're totally right that plugging 3 and 5 into D gives an even number. HOWEVER, plugging 1 and 5 (both odd numbers) into D gives

    (1+5)/2
    6/2
    3

    Which is odd. So D can be odd, which means that it is not correct to say that it must not be odd.

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    Thanked by: Vincen
    Post Tue Sep 26, 2017 11:17 am
    Hi Vincen,

    This question can be approached with Number Property rules. We're told that A and B are both ODD, POSITIVE INTEGERS. We're asked which of the following MUST NOT be odd.

    Answer A: (A)(B) = (odd)(odd) = odd
    Answer B: (A)(A)(B) = (odd)(odd)(odd) = odd
    Answer C: B - A - 1 = (odd) - (odd) - (odd) = odd
    Answer D: (A+ B)/2 = (odd + odd)/2 = (even)/2 = COULD be even (re: 4/2 = 2) OR odd (re: 6/2 = 3)
    Answer E: A - B = (odd) - (odd) = EVEN

    There's only one answer that can NEVER be odd.

    Final Answer: E

    GMAT assassins aren't born, they're made,
    Rich

    _________________
    Contact Rich at Rich.C@empowergmat.com

    GMAT/MBA Expert

    Post Tue Sep 26, 2017 4:50 pm
    You asked about D specifically, so I'll only address that.

    It actually comes down to the remainder of our odd integers when divided by 4. If they have different remainders (one has remainder 1, one has remainder 3), then the average of the two will be even. If they have the same remainder by 4 (both remainder 1 or both remainder 3), then the average will be odd.

    A few examples:

    5 + 7 => 12, average = 6
    5 + 9 => 14, average = 7
    7 + 11 => 11, average = 9

    As a good follow up exercise, see if you can explain why this must be. (Hint: a multiple of 4 can be written as 4*some integer.)

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