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If a and b are positive integers, is a*b a prime number?

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Math Expert
Joined: 02 Sep 2009
Posts: 64221
If a and b are positive integers, is a*b a prime number?  [#permalink]

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06 Dec 2018, 04:40
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Difficulty:

25% (medium)

Question Stats:

70% (00:53) correct 30% (00:44) wrong based on 116 sessions

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If a and b are positive integers, is a*b a prime number?

(1) b is an even number
(2) a < 2

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Status: Attempting to cross the 750 barrier
Joined: 15 Apr 2017
Posts: 6
GMAT 1: 530 Q39 V24
Re: If a and b are positive integers, is a*b a prime number?  [#permalink]

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06 Dec 2018, 04:48
Statement 1
b is an even number
There is no information about a

Insufficient

Statement 2
a<2
From question stem, a is a positive integer => a=1

Insufficient.

Statement 1 and 2 together
Taking both statements together,
a=1 and b is an even number
If b=2, then a*b is a prime number.
If b=4, then a*b is not a prime number.
Therefore, insufficient together as well.

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Re: If a and b are positive integers, is a*b a prime number?  [#permalink]

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06 Dec 2018, 05:17
Bunuel wrote:
If a and b are positive integers, is a*b a prime number?

(1) b is an even number
(2) a < 2

from 1 : b can be any even value not sufficient
from 2; a<2 means a=1 , b not know

from 1 & 2 : b can be any even value and a =1 so ab can be prime at 1*2 and not prime at 1*4 insufficient.
Math Expert
Joined: 02 Sep 2009
Posts: 64221
Re: If a and b are positive integers, is a*b a prime number?  [#permalink]

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24 Dec 2018, 00:19
Bunuel wrote:
If a and b are positive integers, is a*b a prime number?

(1) b is an even number
(2) a < 2

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Re: If a and b are positive integers, is a*b a prime number?   [#permalink] 24 Dec 2018, 00:19