does not represent answer?

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Wed Dec 23, 2009 11:20 pm

XYZW is a four digit number. When reverse of this number, i.e. WZYX is subtracted from it, which of the following does not represent answer?

(A) 999(x - w) +90(y - z)
(B) 999(x - w) +99(y - z)
(C) 9{111(x - w) + 10y - 10z}
(D) 9(111x - 10z) - 9(111w - 19y)
(E) None of these

Note: [spoiler]There is something I would like to complain to the technical team associated with us that I have been observing this from quiet sometime now, that the "minus" sign is giving bizarre results, technically it appears as an asterisk '*' when entered. Any suggestion please?[/spoiler]

Last edited by sanju09 on Wed Dec 23, 2009 11:29 pm, edited 1 time in total.

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Source: Beat The GMAT — Problem Solving | Wed Dec 23, 2009 11:20 pm

papgust: Community Manager; Posts: 1537; Joined: Mon Aug 10, 2009 6:10 pmFollowed by: 252 members; Thanked: 653 times

by papgust » Wed Dec 23, 2009 11:26 pm

sanju09 wrote:XYZW is a four digit number. When reverse of this number, i.e. WZYX is subtracted from it, which of the following does not represent answer?

(A) 999(x - w) +90(y - z)
(B) 999(x - w) +99(y - z)
(C) 9{111(x - w) + 10y - 10z}
(D) 9(111x - 10z) - 9(111w - 19y)
(E) None of these

IMO B

Just plugged in a value (1234) and checked with the choices. I guess there is a better method of doing this.

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linkinpark: Master | Next Rank: 500 Posts; Posts: 124; Joined: Tue Dec 22, 2009 8:32 am; Location: Karjat; Thanked: 3 times

by linkinpark » Thu Dec 24, 2009 11:57 am

I was getting B tried two values

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viidyasagar: Community Manager; Posts: 156; Joined: Mon Oct 22, 2007 6:06 amFollowed by: 3 members; Location: Mumbai, India; Thanked: 16 times; GMAT Score:700

by viidyasagar » Thu Dec 24, 2009 8:01 pm

I think the approach for any "number and it's reverse" problem is as follows

XYZW = 1000*X+100*Y+10*Z+1*W ........1

and

WZYX = 1000*W+100*Z+10*Y+1*X ........2

1 - 2 gives, 999*X+90*Y-90*Z-999*W ......3

Option A,C and D have are mere rearrangements of 3....and it's clear that if A is correct then B can't be correct..

Hence B

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Thu Dec 24, 2009 9:23 pm

sanju09 wrote:XYZW is a four digit number. When reverse of this number, i.e. WZYX is subtracted from it, which of the following does not represent answer?

(A) 999(x - w) +90(y - z)
(B) 999(x - w) +99(y - z)
(C) 9{111(x - w) + 10y - 10z}
(D) 9(111x - 10z) - 9(111w - 19y)
(E) None of these

Picking numbers is a great way to go, but we make our lives much simpler if we notice that A, C and D are all the exact same expression.

Accordingly, we only need to test one choice: A, C or D.

If our choice does represent the answer, then we can eliminate A, C, D and E... B must be correct.

If our choice doesn't represent the answer, then we automatically pick E.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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Mylogin: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Sun Sep 07, 2008 9:21 am; Thanked: 2 times

by Mylogin » Fri Dec 25, 2009 5:04 pm

A quick scan suggests that A & B are different (without even bothering about C & D)

So if you can discount either A or B, you have the answer without bothering about anything else.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Fri Dec 25, 2009 7:44 pm

Mylogin wrote:A quick scan suggests that A & B are different (without even bothering about C & D)

So if you can discount either A or B, you have the answer without bothering about anything else.

How?

There's still answer choice E - so even if you discount B first, the answer could be B or E.

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