gmatclub DS problem

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arora007: Community Manager; Posts: 1048; Joined: Mon Aug 17, 2009 3:26 amFollowed by: 27 members; Location: India; Thanked: 51 times; GMAT Score:670

by arora007 » Tue Jul 06, 2010 7:01 am

Is A + B + C even?

1. A - C - B is even
2. (A - C)/B is odd

OA is E

gmatclub math problem

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Source: Beat The GMAT — Data Sufficiency | Tue Jul 06, 2010 7:01 am

kvcpk: Legendary Member; Posts: 1893; Joined: Sun May 30, 2010 11:48 pmFollowed by: 7 members; Thanked: 215 times

by kvcpk » Tue Jul 06, 2010 7:13 am

Is A + B + C even?

1. A - C - B is even
let A=B=C=0
A+B+C is even
let a=1, b=c=0
A+B+C is odd
INSUFF
2. (A - C)/B is odd
Let (A-C)/B =2k+1
A-C = 2kB+B
B+C = A-2kB
A+B+C=2A-2KB
=2(A-KB)
This is always even

I believe answer should be B

Am i missing something?

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Rich@VeritasPrep: GMAT Instructor; Posts: 147; Joined: Tue Aug 25, 2009 7:57 pmFollowed by: 17 members; Location: New York City; Thanked: 76 times; GMAT Score:770

by Rich@VeritasPrep » Tue Jul 06, 2010 7:56 am

Hey kvcpk,

The tricky thing here is that A, B, and C are not specified as integers.

So, while you showed that A+B+C = 2(A-KB) , we don't know that A and B are integers, so we can't say for sure that the right side of the equation is even.

If A = 2, B = 1, and C = 1, then the conditions of both statements are met, and A+B+C is even.

However, if A = 3/2, B = 1/2, and C = 1, the conditions of both statements are still met, but A+B+C is odd.

Thus the final answer is E

Rich Zwelling
GMAT Instructor, Veritas Prep

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kvcpk: Legendary Member; Posts: 1893; Joined: Sun May 30, 2010 11:48 pmFollowed by: 7 members; Thanked: 215 times

by kvcpk » Tue Jul 06, 2010 8:57 am

raz1024 wrote:Hey kvcpk,

The tricky thing here is that A, B, and C are not specified as integers.

So, while you showed that A+B+C = 2(A-KB) , we don't know that A and B are integers, so we can't say for sure that the right side of the equation is even.

If A = 2, B = 1, and C = 1, then the conditions of both statements are met, and A+B+C is even.

However, if A = 3/2, B = 1/2, and C = 1, the conditions of both statements are still met, but A+B+C is odd.

Thus the final answer is E

oh.. thanks Raz.. missed that..

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nikhilkatira: Master | Next Rank: 500 Posts; Posts: 318; Joined: Mon Jul 13, 2009 3:55 am; Thanked: 12 times

by nikhilkatira » Fri Jul 09, 2010 10:10 am

kvcpk wrote:
raz1024 wrote:Hey kvcpk,

The tricky thing here is that A, B, and C are not specified as integers.

So, while you showed that A+B+C = 2(A-KB) , we don't know that A and B are integers, so we can't say for sure that the right side of the equation is even.

If A = 2, B = 1, and C = 1, then the conditions of both statements are met, and A+B+C is even.

However, if A = 3/2, B = 1/2, and C = 1, the conditions of both statements are still met, but A+B+C is odd.

Thus the final answer is E
oh.. thanks Raz.. missed that..

Hey raz1024

what approach should be used for guessing such problem ? ( if trail and error doesn't work )

Best,
Nikhil H. Katira

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