DS GMATPrep Question

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anwarma: Senior | Next Rank: 100 Posts; Posts: 56; Joined: Sat May 24, 2008 5:18 pm; Thanked: 1 times

by anwarma » Fri Aug 08, 2008 6:37 pm

If M is a finite set of negative integers, is the total numbers of integers in M an odd number?

1) The product of all integers in M is odd

2) The product of all integers in M is negative

OA is B

I thought Answer is E , because we do not know how many negative numbers are in a set M. If they are 3 numbers in a set , it would be B because there is no indication in the question, how many total numbers are in the set M.

Can someone please point out to me why I am wrong?

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Source: Beat The GMAT — Data Sufficiency | Fri Aug 08, 2008 6:37 pm

pepeprepa: Legendary Member; Posts: 661; Joined: Tue Jul 08, 2008 12:58 pm; Location: France; Thanked: 48 times

by pepeprepa » Sat Aug 09, 2008 4:00 am

1) The product of all integers in M is odd
We agree "odd x odd = odd" so we cannot state anything. Insufficient.

2) The product of all integers in M is negative
You do not need to know how many negative numbers are in the set.
You know the total numbers of integers in M is an odd number:
- if the number of negative integers which are multiplied is odd so the result is negative.
- if the number of negative integers which are multiplied is even so the result is positive.

For example,
-3 * -2 * -1 = -6
-4 * -3 * -2 * -1= 24

Hope it's ok.

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anwarma: Senior | Next Rank: 100 Posts; Posts: 56; Joined: Sat May 24, 2008 5:18 pm; Thanked: 1 times

by anwarma » Sat Aug 09, 2008 5:00 am

Thanks! I over analyzed the problem. But your explanation cleared my confusion.

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