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## Probability problem...need explanation plzz

tagged by: Brent@GMATPrepNow

This topic has 2 expert replies and 2 member replies
akpareek Senior | Next Rank: 100 Posts
Joined
14 Feb 2010
Posted:
38 messages

#### Probability problem...need explanation plzz

Wed Sep 04, 2013 11:03 am
M= {-6, -5, -4, -3, -2}
T= {-2, -1, 0, 1, 2, 3}

If an integer is to be randomly selected from set M above and an integer is to be randomly selected from set T above, what is the probability that the product of the two integers will be negative?

A. 0
B. 1/3
C. 2/5
D. 1/2
E. 3/5

### GMAT/MBA Expert

Brent@GMATPrepNow GMAT Instructor
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Wed Sep 04, 2013 11:12 am
akpareek wrote:
M= {-6, -5, -4, -3, -2}
T= {-2, -1, 0, 1, 2, 3}

If an integer is to be randomly selected from set M above and an integer is to be randomly selected from set T above, what is the probability that the product of the two integers will be negative?

A. 0
B. 1/3
C. 2/5
D. 1/2
E. 3/5
IMPORTANT: the number chosen from set M is ALWAYS negative.
So, the only way to get a negative product is when the number chosen from set T is POSITIVE.

P(number chosen from set T is positive) = 3/6 = 1/2 = D

Cheers,
Brent

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vipulgoyal Master | Next Rank: 500 Posts
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Wed Sep 04, 2013 8:39 pm
no is positive = [1 - no is negtive + no is O]
[1- (5/5*2/6 + 5/5*1/6]
1-1/2=1/2

madhusudhan237 Newbie | Next Rank: 10 Posts
Joined
30 Oct 2015
Posted:
5 messages
Thu Dec 24, 2015 7:23 am
Brent@GMATPrepNow wrote:
akpareek wrote:
M= {-6, -5, -4, -3, -2}
T= {-2, -1, 0, 1, 2, 3}

If an integer is to be randomly selected from set M above and an integer is to be randomly selected from set T above, what is the probability that the product of the two integers will be negative?

A. 0
B. 1/3
C. 2/5
D. 1/2
E. 3/5
IMPORTANT: the number chosen from set M is ALWAYS negative.
So, the only way to get a negative product is when the number chosen from set T is POSITIVE.

P(number chosen from set T is positive) = 3/6 = 1/2 = D

Cheers,
Brent
HI Brent

Cannot i solve this problem the following way?

P(that product of 2 integers is -ve) = 1 - P(that product of 2 integers is +ve)
= 1 - [ P(integer selected from set"M" is -ve) AND P(integer selected from set T is -ve)]
= 1 - [ (5/5) * (2/6)]
= 1 - (1/3)
= 2/3

Where am i getting this wrong?

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Rich.C@EMPOWERgmat.com Elite Legendary Member
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Thu Dec 24, 2015 9:13 am

Set T contains a 0, so you ALSO have to factor in the possibility that the product of the two numbers will be 0 (and subtract that outcome as well). Your solution did not include that possibility.

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Rich

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