BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Cards in deck

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 111
Joined: Sat Apr 14, 2012 11:09 am
Thanked: 1 times
Followed by:7 members

Cards in deck

by Soumita Ghosh » Thu Jan 31, 2013 8:05 pm
The cards in a deck are numbered consecutively from 1 to 20. If some cards are red and some
cards are black, what is the probability of drawing a red card?
(1) The odd-numbered cards are all red.
(2) The even-numbered cards are all black.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is
sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient

OA C

I choose D
Top
Source: — Data Sufficiency |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 451
Joined: Thu Jan 21, 2010 11:58 am
Location: New York City
Thanked: 188 times
Followed by:120 members
GMAT Score:770

by Tommy Wallach » Thu Jan 31, 2013 11:38 pm
Hey Soumita,

When you post a question, it might help to explain the logic you used to get to your answer. In this case, you seem to be assuming that:

The odd cards are red = only the odd cards are red

But those aren't the same thing. Statement 1 merely tells you that all the odd-numbered cards are red. But that doesn't mean the majority of the even-numbered cards aren't also red. It could be that JUST ONE even-numbered card is black, and the rest are red. Or maybe ONE is red and the rest are black.

But if you put the two statements together, you know about every single card.

Hope that helps!

-t
Tommy Wallach, Company Expert
ManhattanGMAT

If you found this posting mega-helpful, feel free to thank and/or follow me!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2095
Joined: Tue Dec 04, 2012 3:22 pm
Thanked: 1443 times
Followed by:247 members

by ceilidh.erickson » Fri Feb 01, 2013 3:44 pm
Tommy is right on the money here. I just want to add - this problem is a great example of why you should try to prove insufficiency rather than sufficiency with DS statements. If you're asking yourself "could this statement be enough to answer the question?" you might fall into traps. The trap in this problem is that they want you to assume that "odd cards are red" means "only odd cards are red."

Instead, you should be asking yourself - "how can I prove this insufficient? Could I come up with more than 1 answer to the question, given this statement?" If you had looked at it that way, you probably would have thought of the examples that Tommy gave, and seen that the statements were insufficient on their own.

Another good rule of thumb: if it feels too easy, it probably is! You probably chose D in under a minute on this problem, right? Whenever you get an answer that quickly, step back and ask yourself if there's anything you've overlooked. Are there traps built into the problem?
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 298
Joined: Sun Jun 03, 2012 6:42 am
Location: New delhi
Thanked: 10 times
Followed by:7 members
GMAT Score:590

by varun289 » Fri Feb 01, 2013 4:08 pm
ya it is very good question

even i face same trap , but one leaning for gmat here -

never ever assume / presume any unstated fact,in GMAT as well in real life


use critique method to get out of it/
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2095
Joined: Tue Dec 04, 2012 3:22 pm
Thanked: 1443 times
Followed by:247 members

by ceilidh.erickson » Fri Feb 01, 2013 4:12 pm
never ever assume / presume any unstated fact,in GMAT as well in real life
You're absolutely right! This is perhaps one of the only lessons from the GMAT that you'll actually use in business school :)
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
Top
Master | Next Rank: 500 Posts
Posts: 111
Joined: Sat Apr 14, 2012 11:09 am
Thanked: 1 times
Followed by:7 members

by Soumita Ghosh » Sun Feb 03, 2013 11:41 am
yeah now I understand where I am wrong..

Thanks a lot ceilidh.erickson and Tommy Wallach.
Top
Post new topic Post Reply
• Page 1 of 1