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

Redeem

In a certain class, one student is to be selected at random.

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Data Sufficiency |
User avatar
Community Manager
Posts: 1537
Joined: Mon Aug 10, 2009 6:10 pm
Thanked: 653 times
Followed by:252 members

by papgust » Tue Mar 16, 2010 9:30 pm
(1) 2/3 are boys and 1/3 are girls.

Whenever you have fractions like this, this is sufficient to answer probability.

Let's say class strength is 100, Boys are 66 and girls are 34. Probability that a boy will read is 66/100 = 2/3 approx.
OR
Let's say class strength is 48, Boys are 32 and girls are 16. Probability that a boy will read is 32/48 = 2/3.

Probability will always be same whatever numbers you plug in for this ratio. So, when you have ratios such as this, it will be sufficient to answer.

(2) I guess you are comfortable why (2) is insufficient.
Top
User avatar
Community Manager
Posts: 1537
Joined: Mon Aug 10, 2009 6:10 pm
Thanked: 653 times
Followed by:252 members

by papgust » Tue Mar 16, 2010 9:31 pm
Btw, Moved to Data Sufficiency folder
Top
Legendary Member
Posts: 610
Joined: Fri Jan 15, 2010 12:33 am
Thanked: 47 times
Followed by:2 members

by kstv » Tue Mar 16, 2010 9:52 pm
In a certain class, one student is to be selected at random to read. what's the probability that a boy will read.
(1) Two-thirds or the students in the class are boys
(2) Ten of the students in the class are girls
Probability of an event= Possible ways the event can happen/ Total possibility
To choose a boy we need to know how many boys are there and class size. Mind you the ratio will do.
2/3 say that if the class size is 3 no of boys are 2. Sufficient.
(2) No of boys known, but class size not known. Insufficient.
Top
Senior | Next Rank: 100 Posts
Posts: 65
Joined: Sun Aug 22, 2010 2:55 pm

by siddhans » Tue May 31, 2011 3:43 pm
In second case we know the number of girls are 10 . So can we not find out the number of boys ? which will be 2/3 * 10 = 20 . SO total class is of size 30 and probability that a boy will read is 20/30 = 2/3 ?


Isnt it ? So C should be correct ?? Please explain
Top
Senior | Next Rank: 100 Posts
Posts: 97
Joined: Sun May 15, 2011 9:19 am
Thanked: 18 times
Followed by:1 members

by SoCan » Tue May 31, 2011 4:20 pm
siddhans wrote:In second case we know the number of girls are 10 . So can we not find out the number of boys ? which will be 2/3 * 10 = 20 . SO total class is of size 30 and probability that a boy will read is 20/30 = 2/3 ?


Isnt it ? So C should be correct ?? Please explain
Be careful not to include information from statement 1 when you're evaluating statement 2 on its own. The question stem does not give any information on the size or gender mix. Statement 2 only states that there are 10 girls. You don't have any information about the boys.
Top
Senior | Next Rank: 100 Posts
Posts: 65
Joined: Sun Aug 22, 2010 2:55 pm

by siddhans » Tue May 31, 2011 4:41 pm
But how will C ever be an answer choice then? When do we evaluate both the choices together to find an answer? I am confused
Top
Senior | Next Rank: 100 Posts
Posts: 97
Joined: Sun May 15, 2011 9:19 am
Thanked: 18 times
Followed by:1 members

by SoCan » Tue May 31, 2011 4:49 pm
siddhans wrote:But how will C ever be an answer choice then? When do we evaluate both the choices together to find an answer? I am confused
You should review the basics of DS questions. Do you have the OG or any other prep materials?
Top
Senior | Next Rank: 100 Posts
Posts: 65
Joined: Sun Aug 22, 2010 2:55 pm

by siddhans » Tue May 31, 2011 4:50 pm
Yes, i do...
Top
Senior | Next Rank: 100 Posts
Posts: 97
Joined: Sun May 15, 2011 9:19 am
Thanked: 18 times
Followed by:1 members

In a certain class, one student is to be selected at random.

by SoCan » Tue May 31, 2011 4:53 pm
siddhans wrote:Yes, i do...
I would review that section. This tree is pretty helpful
Image
Top
Senior | Next Rank: 100 Posts
Posts: 65
Joined: Sun Aug 22, 2010 2:55 pm

by siddhans » Wed Jun 01, 2011 8:44 am
Thanks i reviewed and practiced some questions and got the hang of it now ;) thanks everyone
Top
User avatar
GMAT Instructor
Posts: 1462
Joined: Thu Apr 09, 2015 9:34 am
Location: New York, NY
Thanked: 39 times
Followed by:22 members

by Jeff@TargetTestPrep » Tue Jul 17, 2018 3:47 pm
TheGuest wrote:In a certain class, one student is to be selected at random to read. what's the probability that a boy will read.

(1) Two-thirds or the students in the class are boys
(2) Ten of the students in the class are girls
We need to determine the probability that a boy will be selected to read, in a class of boys and girls.

Statement One Alone:

Two-thirds of the students in the class are boys.

Since we know that 2/3 of the class are boys, we know that the probability of randomly selecting a boy to read is 2/3. Statement one alone is sufficient to answer the question.
Statement Two Alone:

Ten of the students in the class are girls.

Without knowing the number of boys in the class, we cannot determine the probability that a boy will be selected to read. Statement two alone is not sufficient to answer the question.

Answer: A

Jeffrey Miller
Head of GMAT Instruction
[email protected]

Image

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews
Top
Post new topic Post Reply
• Page 1 of 1