Problem with a question from GMATPrep

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Kevinst: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Tue Feb 28, 2012 8:57 am; Thanked: 11 times; Followed by:6 members

Problem with a question from GMATPrep

by Kevinst » Tue Aug 21, 2012 1:19 pm

In the xy-plane is the slope of line k equal to 0?

1) The x-intercept of k is 0
2) The y-intercept of k is 0.

The proposed answer is E

However, I assumed it should be A

GMAT's reasoning regarding (1):
If line k has equation y=0, then it has x-intercept equal to 0 and its slope is 0. On the other hand, if line k has equation y = x, then it has x-intercept equal to 0 but its slope is 1.

I disagree with that reasoning as a line y = 0 doesn't have a real x-intercept. The whole x-axis is intercepted. Nothing makes point (0,0) better as a x-intercept than (5,0).
As 1) clearly states that we do have a valid x-intercept, I assumed that our line must have a slope different from 0.

Thoughts?

Thanks,
Kevin

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Source: Beat The GMAT — Data Sufficiency | Tue Aug 21, 2012 1:19 pm

rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 am; Location: New Delhi, India; Thanked: 130 times; Followed by:9 members; GMAT Score:720

by rijul007 » Tue Aug 21, 2012 2:42 pm

Kevinst wrote: I disagree with that reasoning as a line y = 0 doesn't have a real x-intercept. The whole x-axis is intercepted. Nothing makes point (0,0) better as a x-intercept than (5,0).
As 1) clearly states that we do have a valid x-intercept, I assumed that our line must have a slope different from 0.

Thoughts?

Thanks,
Kevin

line y=0 has infinite x-intercepts
0 can also be considered as an x-intercept
so in this case is slope of line 0?
Yes

consider line y = x
It has x-intercept = 0
Is the slope equal to 0?
No

Hence the statement is Insufficient

Hope this clears the doubt.

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Kevinst: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Tue Feb 28, 2012 8:57 am; Thanked: 11 times; Followed by:6 members

by Kevinst » Tue Aug 21, 2012 3:11 pm

rijul007 wrote:
Kevinst wrote: I disagree with that reasoning as a line y = 0 doesn't have a real x-intercept. The whole x-axis is intercepted. Nothing makes point (0,0) better as a x-intercept than (5,0).
As 1) clearly states that we do have a valid x-intercept, I assumed that our line must have a slope different from 0.

Thoughts?

Thanks,
Kevin
line y=0 has infinite x-intercepts
0 can also be considered as an x-intercept
so in this case is slope of line 0?
Yes

consider line y = x
It has x-intercept = 0
Is the slope equal to 0?
No

Hence the statement is Insufficient

Hope this clears the doubt.

I understand your point. Though, I am still not convinced as the wording is "The x-intercept of k is 0". "The" to me means "The one and only". It would have been a different story if they wrote "A x-intercept of k is 0."

But I agree, I am probably too picky and interpret too much into GMAT's choice of words.
I just hope that I won't see a similar ambiguity on D-Day.

Thanks

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

by Ian Stewart » Tue Aug 21, 2012 6:41 pm

I can count only three official published problems I've ever seen (among many thousands) that I find mathematically questionable. There's one problem that appeared in an older version of GMATPrep which had the wrong answer (they forgot about negative numbers!), one DS problem which can be solved without any statements at all (an absolute value/inequality question that has been posted on BTG many times), and then there's this problem.

I entirely agree with Kevinst that it makes no sense to talk about the x-intercept of the line y=0. So I find it perfectly reasonable here to think Statement 1 is sufficient - it appears to rule out the possibility that the line is horizontal, so it seems sufficient to give a "no" answer to the question. But of course if you consider x=0 to be 'the' x-intercept of y=0, it isn't sufficient.

While this question is pretty terrible, you can at least take comfort in the fact that the number of bad official questions is negligibly small. So you are almost certain not to encounter this kind of ambiguity on your actual test. And if your answer to this question was different from theirs, don't worry about it.

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

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