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VivianKerr: GMAT Instructor; Posts: 1035; Joined: Fri Dec 17, 2010 11:13 am; Location: Los Angeles, CA; Thanked: 474 times; Followed by:365 members

GMATPrep Data Sufficiency - new one!

by VivianKerr » Wed Jul 15, 2015 11:01 am

I hadn't seen this l'il guy posted and discussed before, so I wanted to share it with you all! I believe it is from GMATPrep 3, just reviewed it with a student. Sharpen your knives...

Of the numbers r+s, r-s, rs, and r/s, which is the greatest?

(1) r = s = 1
(2) r - s is the least

OA to follow!

Vivian Kerr
GMAT Rockstar, Tutor
https://www.GMATrockstar.com
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Former Kaplan and Grockit instructor, freelance GMAT content creator, now offering affordable, effective, Skype-tutoring for the GMAT at $150/hr. Contact: [email protected]

Thank you for all the "thanks" and "follows"!

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Source: Beat The GMAT — Data Sufficiency | Wed Jul 15, 2015 11:01 am

prachi18oct: Master | Next Rank: 500 Posts; Posts: 269; Joined: Sun Apr 27, 2014 10:33 pm; Thanked: 8 times; Followed by:5 members

by prachi18oct » Wed Jul 15, 2015 1:43 pm

IMO A

(1) r = s = 1 => r+s = 2, r-s = 0, rs = 1, r/s = 1 r+s is greatest. SUFFICIENT
(2) r-s is least.

Since r-s is least , r+s > r-s => s>0
We have to check cases for r;
r > 0 , Let r = 3 s = 2
r+s = 5, r-s = 1 , rs = 6 , r/s = 3/2 => rs is greatest
Let r = 1/2 , s = 1/3
r+s = 5/6 , r-s = 1/6 , rs = 1/6, r/s = 3/2 => r+s is greatest here.
Different answers => NOT SUFFICIENT

Hence A

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

by Brent@GMATPrepNow » Thu Jul 16, 2015 8:11 pm

VivianKerr wrote:
Of the numbers r+s, r-s, rs, and r/s, which is the greatest?

(1) r = s = 1
(2) r - s is the least

Target question: Of the numbers r+s, r-s, rs, and r/s, which is the greatest?

Statement 1: r = s = 1
Beautiful, we're given the actual values of the only 2 variables.
r+s = 2, r-s = 0, rs = 1, and r/s = 1
r+s has the greatest value
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: r - s is the least
This statement doesn't FEEL sufficient, so I'm going to TEST some values.
Of course, we need to ensure that r-s has the smallest value.
One way to do this is to make r and s both positive AND make r < s. Doing so, will make r-s NEGATIVE, while the other three expressions will be positive. This ensures that r-s has the smallest value
Here are two possible cases that yield conflicting answers to the target question:
Case a: r = 1 and s = 2. Here, r+s = 3, r-s = -1, rs = 2, and r/s = 1/2. In this case r+s has the greatest value
Case b: r = 2 and s = 3. Here, r+s = 5, r-s = -1, rs = 6, and r/s = 2/3. In this case rs has the greatest value
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Aside: For more on this idea of plugging in values when a statement doesn't feel sufficient, you can read my article: https://www.gmatprepnow.com/articles/dat ... lug-values

Answer = A

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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VivianKerr: GMAT Instructor; Posts: 1035; Joined: Fri Dec 17, 2010 11:13 am; Location: Los Angeles, CA; Thanked: 474 times; Followed by:365 members