Conference Room

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Abdulla: Master | Next Rank: 500 Posts; Posts: 328; Joined: Thu Aug 07, 2008 5:25 pm; Location: Philadelphia; Thanked: 4 times; GMAT Score:550

by Abdulla » Sat Dec 13, 2008 8:05 pm

In a certain conference room each row of chairs has the same number of chairs, and the number of rows is 1 less than the number of chairs in a row. How many chairs are in the row?

(1) There is a total of 72 chairs.
(2) After one chair is removed from the last row, there is a total of 17 chairs in the last two rows.

OA is D

Last edited by Abdulla on Sat Dec 13, 2008 10:00 pm, edited 1 time in total.

Abdulla

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Source: Beat The GMAT — Data Sufficiency | Sat Dec 13, 2008 8:05 pm

ronniecoleman: Legendary Member; Posts: 546; Joined: Sun Nov 16, 2008 11:00 pm; Location: New Delhi , India; Thanked: 13 times

by ronniecoleman » Sat Dec 13, 2008 9:10 pm

1: N*(N-1) = 72

N = -8 , 9

N cannot be negative hence n = 9

2.

ADD 1 to 17 = 18
each row will have 18/2 = 9

hence we have 9 chairs

IMO D

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sat Dec 13, 2008 9:22 pm

let the rows be R
let the chairs in each row be ch

r = ch-1

Stmt I

ch * (ch-1) = 72

Possible value ch = 9 ch-1 = 8

9 chairs

SUFF

Stmt II

one chair removed makes it 17 so there were 18 chair in2 rows before removing the 1 chair

Again 9 chairs

SUFF

D)

Last edited by cramya on Sat Dec 13, 2008 9:43 pm, edited 1 time in total.

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sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

Re: Conference Room

by sudhir3127 » Sat Dec 13, 2008 9:34 pm

Abdulla wrote:In a certain conference room each row of chairs has the same number of chairs, and the number of rows is 1 less than the number of chairs in a row. How many chairs are in the row?

(1) There is a total of 72 chairs.
(2) After one chair is removed from the last row, there is a total of 17 chairs in the last two rows.

OA is C

i think this is a gmat prep question and the OA has to be D . Please make sure and doubly chk the questions and Answers before u post, It unncessarily creates confusion and takes a lot of time .

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Abdulla: Master | Next Rank: 500 Posts; Posts: 328; Joined: Thu Aug 07, 2008 5:25 pm; Location: Philadelphia; Thanked: 4 times; GMAT Score:550

Re: Conference Room

by Abdulla » Sat Dec 13, 2008 9:59 pm

sudhir3127 wrote:
Abdulla wrote:In a certain conference room each row of chairs has the same number of chairs, and the number of rows is 1 less than the number of chairs in a row. How many chairs are in the row?

(1) There is a total of 72 chairs.
(2) After one chair is removed from the last row, there is a total of 17 chairs in the last two rows.

OA is C
i think this is a gmat prep question and the OA has to be D . Please make sure and doubly chk the questions and Answers before u post, It unncessarily creates confusion and takes a lot of time .

I'm so sorry.. it was my mistake .. i didn't mean it.

Abdulla

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vittalgmat: Legendary Member; Posts: 621; Joined: Wed Apr 09, 2008 7:13 pmFollowed by: 4 members; Thanked: 33 times

by vittalgmat » Sat Dec 13, 2008 11:52 pm

Yep got it .
I used one my take aways to reduce calculation and here it is.
(I am not sure if I can apply in all cases. So pls review and let me
know your feedback.)

Let the number of rows be r
stmt 1: says
r(r+1) = 72.
=> r^2 +r -72 = 0
72 is not a perfect square. So the two roots will be of opposite sign.
Dont care what it is. one of them is +ve which is what I need coz the
chairs and/or rows cannot be -ve. Therefore sufficient.
As a side note: I wonder if I can make the above conclusion.. what if the two roots are -ve something and a +ve fraction. Then both are invalid coz we cannot have fractional rows or chairs.
Am I taking a risk on the GMAT question by assuming that one of the roots will always be 'practical'??

Some one pls comment.

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Dec 14, 2008 12:00 am

Am I taking a risk on the GMAT question by assuming that one of the roots will always be 'practical'??

Hard to tell.

Lets say eqn is x^2+6x+6 (6 is not a perfect square)

One distinct root but -ve.

IMO I would spend the extra 30 secs - minute to make sure it leads to the desired root to predict sufficiency.

Strictly my opinion.Hope I dint miss something in your logic above.

Regards,
Cramya

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pbanavara: Master | Next Rank: 500 Posts; Posts: 279; Joined: Wed Sep 24, 2008 8:26 am; Location: Portland, OR; Thanked: 6 times

by pbanavara » Sun Dec 14, 2008 12:34 am

vittalgmat wrote:Yep got it .
I used one my take aways to reduce calculation and here it is.
(I am not sure if I can apply in all cases. So pls review and let me
know your feedback.)

Let the number of rows be r
stmt 1: says
r(r+1) = 72.
=> r^2 +r -72 = 0
72 is not a perfect square. So the two roots will be of opposite sign.
Dont care what it is. one of them is +ve which is what I need coz the
chairs and/or rows cannot be -ve. Therefore sufficient.
As a side note: I wonder if I can make the above conclusion.. what if the two roots are -ve something and a +ve fraction. Then both are invalid coz we cannot have fractional rows or chairs.
Am I taking a risk on the GMAT question by assuming that one of the roots will always be 'practical'??

Some one pls comment.

Honestly I wouldn't risk it .. when u r at this stage - it will take another 10 seconds to figure out the roots and the answer. May be 15 but I would spend the 15 secs and be sure of my answer.

- pradeep

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vittalgmat: Legendary Member; Posts: 621; Joined: Wed Apr 09, 2008 7:13 pmFollowed by: 4 members; Thanked: 33 times

by vittalgmat » Sun Dec 14, 2008 1:07 am

Thank you guys.
I will use it with utmost care.. and may be even abandon this.
good point.

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vishubn: Legendary Member; Posts: 574; Joined: Sun Jun 01, 2008 8:48 am; Location: Bangalore; Thanked: 28 times

Re: Conference Room

by vishubn » Sun Dec 14, 2008 1:17 am

Abdulla wrote:
sudhir3127 wrote:
Abdulla wrote:In a certain conference room each row of chairs has the same number of chairs, and the number of rows is 1 less than the number of chairs in a row. How many chairs are in the row?

(1) There is a total of 72 chairs.
(2) After one chair is removed from the last row, there is a total of 17 chairs in the last two rows.

OA is C
i think this is a gmat prep question and the OA has to be D . Please make sure and doubly chk the questions and Answers before u post, It unncessarily creates confusion and takes a lot of time .

In a way its FUN

One will not prove to get to the OA rather prove it to be right

Vishu
I'm so sorry.. it was my mistake .. i didn't mean it.

KILL !! DIE !! or BEAT my FEAR !!! de@D END!!

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vladmire: Master | Next Rank: 500 Posts; Posts: 132; Joined: Tue Oct 07, 2008 4:59 pm; Thanked: 4 times

Rows

by vladmire » Sun Dec 21, 2008 11:40 am

Can anyone explain how statement one is sufficient by itself? ***note** the number of rows is 1 less than the number of chairs in a row.

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