Is (a/b)>(3/5)?
1) (a+1/b)>(3/5)
2) (a/b+1)>(3/5)
Answer: B
5-Day Free Trial
5-day free, full-access trial TTP
Available with Beat the GMAT members only code
MORE DETAILS
Fraction Question
This topic has expert replies
GMAT/MBA Expert
-
[email protected]
- Elite Legendary Member
- Posts: 10392
- Joined: Sun Jun 23, 2013 6:38 pm
- Location: Palo Alto, CA
- Thanked: 2867 times
- Followed by:511 members
- GMAT Score:800
Hi topspin20,
This DS question has a great built-in "comparison" shortcut.
The question asks " Is A/B > 3/5?" This is a Yes/No question, so we have to keep track of if the answer is ALWAYS yes, ALWAYS no or Inconsistent.
Let's cross-multiply to make this question easier: "Is 5A > 3B?"
Fact 1 - let's cross-multiply this inequality:
5A + 5 > 3B
We know for sure that 5A + 5 is greater than 3B
MAYBE 5A is greater than 3B, but we have no way to know for sure. By comparison, MAYBE the answer is YES, MAYBE it's NO.
Inconsistent = INSUFFICIENT.
Fact 2 - let's cross-multiply this too:
5A > 3B + 3
Here we know that 5A is greater than 3B + 3, which by definition is greater than 3B. Visually:
5A > 3B + 3 > 3B
So, the comparison proves it - we know that 5A is greater than 3B.
Fact 2 is SUFFICIENT.
Final answer: B
GMAT assassins aren't born, they're made,
Rich
This DS question has a great built-in "comparison" shortcut.
The question asks " Is A/B > 3/5?" This is a Yes/No question, so we have to keep track of if the answer is ALWAYS yes, ALWAYS no or Inconsistent.
Let's cross-multiply to make this question easier: "Is 5A > 3B?"
Fact 1 - let's cross-multiply this inequality:
5A + 5 > 3B
We know for sure that 5A + 5 is greater than 3B
MAYBE 5A is greater than 3B, but we have no way to know for sure. By comparison, MAYBE the answer is YES, MAYBE it's NO.
Inconsistent = INSUFFICIENT.
Fact 2 - let's cross-multiply this too:
5A > 3B + 3
Here we know that 5A is greater than 3B + 3, which by definition is greater than 3B. Visually:
5A > 3B + 3 > 3B
So, the comparison proves it - we know that 5A is greater than 3B.
Fact 2 is SUFFICIENT.
Final answer: B
GMAT assassins aren't born, they're made,
Rich
-
kevincanspain
- GMAT Instructor
- Posts: 613
- Joined: Thu Mar 22, 2007 6:17 am
- Location: madrid
- Thanked: 171 times
- Followed by:64 members
- GMAT Score:790
Let's cross-multiply to make this question easier: "Is 5A > 3B?"
Doing so can cause trouble if variables are negative!
Doing so can cause trouble if variables are negative!
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
GMAT Instructor
Gmatclasses
Madrid
-
GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
Given a/b > 3/5, we can rephrase the question stem as 5a > 3b ONLY IF THE PROBLEM IS RESTRICTED TO POSITIVE VALUES.topspin20 wrote:Is (a/b)>(3/5)?
1) (a+1/b)>(3/5)
2) (a/b+1)>(3/5)
Answer: B
Here, it's possible that b<0, in which case statement 2 is NOT sufficient.
Statement 2: a/(b+1) > 3/5
Case 1: a=4 and b=3
The constraint that a/(b+1) > 3/5 is satisfied:
a/(b+1) = 4/(3+1) = 4/4 = 1.
In this case, a/b = 4/3, so a/b > 3/5.
Case 2: a=0.5 and b=-0.5
The constraint that a/(b+1) > 3/5 is satisfied:
a/(b+1) = 0.5/(-0.5+1) = 0.5/0.5 = 1.
In this case, a/b = 0.5/-0.5 = -1, so a/b < 3/5.
INSUFFICIENT.
Note the following:
In Case 2, a/b < 3/5, but 5a > 3b.
This illustrates why -- if the signs of a and b are unknown -- it's best not to rephrase a/b > 3/5 as 5a > 3b.
If the problem has been posted correctly, the OA should not be B.
What is the source?
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Thank you all for the responses. Mitch, B would be the correct answer if we specify that a and b are positive, correct?
I made the question up to help me to review this concept. I certainly appreciate the correction-which allowed me to learn even more.
I made the question up to help me to review this concept. I certainly appreciate the correction-which allowed me to learn even more.
GMAT/MBA Expert
-
[email protected]
- Elite Legendary Member
- Posts: 10392
- Joined: Sun Jun 23, 2013 6:38 pm
- Location: Palo Alto, CA
- Thanked: 2867 times
- Followed by:511 members
- GMAT Score:800
Hey All,
Both Kevin and Mitch are right on; I goofed that one. Both A and B COULD be negative (or even fractions/decimals). There's a lesson in there about not answering questions too late at night and not after having a couple of drinks. But the point is important and worth noting: when you have no information about a variable in a DS question, then that variable could be ANYTHING.
-Rich
Both Kevin and Mitch are right on; I goofed that one. Both A and B COULD be negative (or even fractions/decimals). There's a lesson in there about not answering questions too late at night and not after having a couple of drinks. But the point is important and worth noting: when you have no information about a variable in a DS question, then that variable could be ANYTHING.
-Rich
