DS question: Instructor, can you please clear this doubt?

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jrikhraj: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Wed Jun 23, 2010 8:02 pm

by jrikhraj » Sat Sep 11, 2010 7:45 am

What is the ratio of y to x?

(1) y = 3x

(2) x = 11

Can we divide the equation in (1) by x to get the ratio as 3 even if we are not explicitly given that x!=0?

The OA is A. However, I marked C since we were not given that x is !=0.

Instructor, please clear this doubt so that I don't get confused in such an easy problem again !

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Source: Beat The GMAT — Data Sufficiency | Sat Sep 11, 2010 7:45 am

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Sat Sep 11, 2010 7:58 am

jrikhraj wrote:What is the ratio of y to x?

(1) y = 3x

(2) x = 11

Can we divide the equation in (1) by x to get the ratio as 3 even if we are not explicitly given that x!=0?

The OA is A. However, I marked C since we were not given that x is !=0.

Instructor, please clear this doubt so that I don't get confused in such an easy problem again !

If this were a real GMAT question, they would tell you in advance that x and y are positive. As you've presented the question, I suppose the right answer ought to be C, because, as you point out, x and y could be zero using statement 1 alone, in which case the ratio would be undefined. You'd never need to worry about an exception like that on a real GMAT question however.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

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Aman verma: Master | Next Rank: 500 Posts; Posts: 304; Joined: Wed Jan 27, 2010 8:35 amFollowed by: 3 members; Location: International Space Station; Thanked: 11 times

by Aman verma » Sat Sep 11, 2010 8:18 am

Yes , Ian has rightly pointed out that on the actual GMAT, the question will mention whether x & y are positive. And , even if we are not explicitly given x! = 0 , if we assume x!=0 , then from the first statement we get a ratio which becomes undefined. So, following Ian's logic , the answer IMOC.

800. Arjun's-Bird-Eye

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jrikhraj: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Wed Jun 23, 2010 8:02 pm

by jrikhraj » Sat Sep 11, 2010 10:46 am

Thanks Ian and Aman

This was a question from one of Manhattan's practice GMAT tests so I assumed that it would be very similar to the real GMAT. Even I was very surprised to see that x>0 was not provided. Anyway, i would take it that my reasoning is correct.

Thanks for the quick reply !!

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