Target Question & DS question

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ashyacid: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Wed Dec 30, 2015 5:54 am

by ashyacid » Sat Jan 02, 2016 11:46 pm

I was going through the DS videos on gmatprepnow, and in this video https://www.gmatprepnow.com/module/gmat- ... video/1100
he said that
Q: Is 4x < 3x
1. 3x + 4 < 3
2. 2x - 3 < 2

And he gave the answer that just 1 is sufficient. But in the previous videos it was told that we need to check the sufficiency, isn't the 2nd statement also sufficient enough to guess if x is negative or not?
What is actually the answer to this question?

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Source: Beat The GMAT — Data Sufficiency | Sat Jan 02, 2016 11:46 pm

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by [email protected] » Sun Jan 03, 2016 9:53 am

Hi ashyacid,

This DS question is based on certain Number Property rules. You might find it easier to deal with if you 'rewrite' the question though...

We're asked if 4X < 3X....

Subtracting 3X from both sides 'rewrites' the question into...

Is X < 0? This is a YES/NO question.

1) 3X+4 < 3

3X < -1
X < -1/3

This Fact proves that X is ALWAYS NEGATIVE, so the answer to the question is ALWAYS YES.
Fact 1 is SUFFICIENT

2) 2X-3 < 2

2X < 5
X < 5/2

This Fact shows that X could be positive OR 0 OR negative. If X is positive or 0, then the answer to the question is NO. If X is negative, then the answer to the question is YES.
Fact 2 is INSUFFICIENT.

Rewriting the question isn't a necessary step to solving this question (the above explanations still apply if you leave the question unchanged), but you should look for opportunities to do so in the future.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Jan 03, 2016 3:12 pm

ashyacid wrote:I was going through the DS videos on gmatprepnow, and in this video https://www.gmatprepnow.com/module/gmat- ... video/1100
he said that
Q: Is 4x < 3x
1. 3x + 4 < 3
2. 2x - 3 < 2

And he gave the answer that just 1 is sufficient. But in the previous videos it was told that we need to check the sufficiency, isn't the 2nd statement also sufficient enough to guess if x is negative or not?
What is actually the answer to this question?

Hi ashyacid,

What do you mean by "...in the previous videos it was told that we need to check the sufficiency?"

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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ashyacid: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Wed Dec 30, 2015 5:54 am

by ashyacid » Mon Jan 04, 2016 12:08 am

Brent@GMATPrepNow wrote:
ashyacid wrote:I was going through the DS videos on gmatprepnow, and in this video https://www.gmatprepnow.com/module/gmat- ... video/1100
he said that
Q: Is 4x < 3x
1. 3x + 4 < 3
2. 2x - 3 < 2

And he gave the answer that just 1 is sufficient. But in the previous videos it was told that we need to check the sufficiency, isn't the 2nd statement also sufficient enough to guess if x is negative or not?
What is actually the answer to this question?
Hi ashyacid,

What do you mean by "...in the previous videos it was told that we need to check the sufficiency?"

Cheers,
Brent

I was referring to this question. Because, we aren;t really getting an answer here either, but still the first statement is sufficient.

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Mon Jan 04, 2016 10:27 am

Sorry, I'm not sure what you're asking.
For your original question, the video explains that the answer is A.
Here's the screenshot:

Statement 2 tells us that x < 2.5
So, x could equal 1, in which case x is NOT negative
OR x could equal -1, in which case x IS negative
Since we cannot answer the rephrased target question with certainty, statement 2 is NOT sufficient.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Amrabdelnaby: Master | Next Rank: 500 Posts; Posts: 137; Joined: Fri Nov 13, 2015 11:01 amFollowed by: 2 members; Thanked: 1 times

by Amrabdelnaby » Thu Jan 07, 2016 1:11 pm

The only possible way for 4x to be less than 3x is when x is -ve.

so lets look at statement 2 2x - 3 < 2

after adding three to both sides u get 2x less than 5 which means that x is less than 2.5

x here can be any number less than 2.5, positive such as 1 or 2 or even a fraction or negative.

if x is positive then the answer to the question is no, and if x is -ve then the answer is yes. hence insufficient

ashyacid wrote:I was going through the DS videos on gmatprepnow, and in this video https://www.gmatprepnow.com/module/gmat- ... video/1100
he said that
Q: Is 4x < 3x
1. 3x + 4 < 3
2. 2x - 3 < 2

And he gave the answer that just 1 is sufficient. But in the previous videos it was told that we need to check the sufficiency, isn't the 2nd statement also sufficient enough to guess if x is negative or not?
What is actually the answer to this question?

Like this post

Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members; Location: East Bay all the way; Thanked: 625 times; GMAT Score:780

by Matt@VeritasPrep » Fri Jan 08, 2016 1:41 pm

Brent@GMATPrepNow wrote:Sorry, I'm not sure what you're asking.

I *think* he means that even if S1 is sufficient alone, you still need to check whether S2 is sufficient alone, but I'm not certain.

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