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Math Expert V
Joined: 02 Sep 2009
Posts: 64068
If positive integers a and b are both odd, which of the following must  [#permalink]

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Difficulty:   25% (medium)

Question Stats: 68% (00:58) correct 32% (00:56) wrong based on 227 sessions

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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

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Manager  B
Joined: 02 Nov 2013
Posts: 67
Location: India
Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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1
If positive integers a and b are both odd.
A. ab ----- Odd * Odd = Odd
B. a(ab)-- Odd * Odd * Odd = Odd
C. b – a – 1 -- Odd - Odd - 1 = Odd
D. (a + b)/2 -- Odd + Odd / 2 = Odd
E. a – b ---- Odd - Odd = Even in all the cases. Hence answer is E.
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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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Bunuel 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

A. ODD * ODD = ODD

B. ODD(ODD*ODD) = ODD

C. ODD - ODD = EVEN - 1 = ODD

D. ODD + ODD = EVEN/2 = EVEN/ODD

E. ODD - ODD = EVEN

So, Correct answer will be (E)
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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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1
Hi All,

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.

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GRE 1: Q169 V154 Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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Odd -Odd => Even
Hence E can never be even.

Hence E

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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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Option E

a & b are Odd integers. Odd: O, Even: E & Fraction: F.

A. ab = O*O = O
B. a(ab) = O(O*O) = O
C. b – a – 1 = O - O - 1 = E - 1 = O
D. (a + b)/2 = (O + O)/2 = E/2 = O
E. a – b = O - O = E B
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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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A. o1*o2=o
B. o1(o1*o2)=o
C. o2-o1-1=o
D. (o1+o2)/2=o
E. o1-o2=e

Hence E
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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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How come D is not the answer if we apply 3+5/2=4 , therefore even?

Bunuel 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
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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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Yoohanna wrote:
How come D is not the answer if we apply 3+5/2=4 , therefore even?

Bunuel 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

Good observation, thats why I mentioned the result of (D) may/may not be EVEN
Abhishek009 wrote:
D. ODD + ODD = EVEN/2 = EVEN/ODD

You have considered the numbers as 3 and 5 , see if I take the numbers as 1 and 5

$$\frac{(a + b)}{2} = \frac{6}{2} = 3 (ODD)$$

Thus, (D) may/may not be even...

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Abhishek....

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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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Bunuel 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

Since odd - odd = even, answer choice E, a - b is not odd.

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Re: If positive integers a and b are both odd, which of the following must  [#permalink]

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_________________ Re: If positive integers a and b are both odd, which of the following must   [#permalink] 09 Mar 2020, 02:16

# If positive integers a and b are both odd, which of the following must  