A flaw in Manhattan GMAT question?

This topic has expert replies
Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Sun Feb 02, 2014 5:16 am

A flaw in Manhattan GMAT question?

by truck » Sun Feb 02, 2014 5:36 am
I find a very confusing question in Manhattan's algebra strategy book pg 115. Please Correct me if Im wrong.

if a=3bc, what is the value of c?
(1) a=10-b
(2) 3a = 4b
Manhattan answer: [spoiler]B: statement 2 itself is sufficient because : 3a=4b => a/b=4/3 => c= 1/3 x a/b = 1/3 * 4/3
[/spoiler]


However I think B is wrong, Because statement 2 leads to 2 results: a/b=4/3 OR b=a=0
in case a=b=0 => c can be any number => correct answer must be C: both statement together are sufficient but neither statement alone is sufficient.

GMAT/MBA Expert

User avatar
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

by [email protected] » Sun Feb 02, 2014 1:20 pm
Hi truck,

You are absolutely correct. For the answer to be B, an additional piece of information would need to be included, namely that none of the variables could equal 0. So there's either a typo in the question or the author made a mistake (and didn't account for the possibility of 0). Since GMAC goes through a rather rigorous testing process, the questions that you'll see on your Official GMAT won't have these sorts of errors in them.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image

User avatar
GMAT Instructor
Posts: 1248
Joined: Thu Mar 29, 2012 2:57 pm
Location: Everywhere
Thanked: 503 times
Followed by:192 members
GMAT Score:780

by Bill@VeritasPrep » Sun Feb 02, 2014 2:32 pm
A perfect example of why "what about 0?" is an important thought in DS.
Join Veritas Prep's 2010 Instructor of the Year, Matt Douglas for GMATT Mondays

Visit the Veritas Prep Blog

Try the FREE Veritas Prep Practice Test

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Sun Feb 02, 2014 5:16 am

by truck » Mon Feb 03, 2014 1:18 am
Thank you so much, your response makes me feel much more confident now.
Cheers!
[email protected] wrote:Hi truck,

You are absolutely correct. For the answer to be B, an additional piece of information would need to be included, namely that none of the variables could equal 0. So there's either a typo in the question or the author made a mistake (and didn't account for the possibility of 0). Since GMAC goes through a rather rigorous testing process, the questions that you'll see on your Official GMAT won't have these sorts of errors in them.

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