Michigan Ross Chat (US calls are expected today) | UCLA Anderson Chat (Calls expected to start at 7am PST; Applicants from Asia will hear first)

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Join our live webinar and learn how to approach Data Sufficiency and Critical Reasoning problems, how to identify the best way to solve each question and what most people do wrong.

From Dec 5th onward, American programs will start releasing R1 decisions. Chat Rooms: We have also assigned chat rooms for every school so that applicants can stay in touch and exchange information/update during decision period.

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

12 Oct 2013, 11:41

4

If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

\(P(b)=\frac{b}{b+g}=?\)

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> \(\frac{b-25}{(b-25)+g}=\frac{3}{4}\) --> \(b-3g=25\). Not sufficient.

(2) There are 35 more boys than there are girls --> \(g=b-35\). Not sufficient.

(1)+(2) We have two linear equation with two unknowns (\(b-3g=25\) and \(g=b-35\)), thus we can solve for both and get the value of \(\frac{b}{b+g}\). Sufficient.

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

12 Oct 2013, 11:49

Bunuel wrote:

\(P(b)=\frac{b}{b+g}=?\)

Thanks, Bunuel. Could you please clarify how the statement 1 "...the probability of selecting a boy will be 0.75" is different from the question itself "what is the probability that the child will be a boy". I'm stuck here because to me it looks like they provide the same information.

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

12 Oct 2013, 11:51

1

LinaNY wrote:

Bunuel wrote:

\(P(b)=\frac{b}{b+g}=?\)

Thanks, Bunuel. Could you please clarify how the statement 1 "...the probability of selecting a boy will be 0.75" is different from the question itself "what is the probability that the child will be a boy". I'm stuck here because to me it looks like they provide the same information.

(1) says that "IF 25 boys are removed from the school, the probability of selecting a boy will be 0.75"
_________________

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

08 Nov 2013, 04:33

Bunuel wrote:

If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

\(P(b)=\frac{b}{b+g}=?\)

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> \(\frac{b-25}{(b-25)+g}=\frac{3}{4}\) --> \(b-3g=25\). Not sufficient.

(2) There are 35 more boys than there are girls --> \(g=b-35\). Not sufficient.

(1)+(2) We have two linear equation with two unknowns (\(b-3g=25\) and \(g=b-35\)), thus we can solve for both and get the value of \(\frac{b}{b+g}\). Sufficient.

Answer: C.

Hi Bunuel,

Can you please shed some light on why it would not be correct to state the following:

For the statement 1, p(girl)=(g/(b-25+g))=0.25.

Assuming this inference is correct, we can find the number of boys using a two equation,two unknowns approach.

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

08 Nov 2013, 04:48

2

Pmar2012 wrote:

Bunuel wrote:

If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

\(P(b)=\frac{b}{b+g}=?\)

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> \(\frac{b-25}{(b-25)+g}=\frac{3}{4}\) --> \(b-3g=25\). Not sufficient.

(2) There are 35 more boys than there are girls --> \(g=b-35\). Not sufficient.

(1)+(2) We have two linear equation with two unknowns (\(b-3g=25\) and \(g=b-35\)), thus we can solve for both and get the value of \(\frac{b}{b+g}\). Sufficient.

Answer: C.

Hi Bunuel,

Can you please shed some light on why it would not be correct to state the following:

For the statement 1, p(girl)=(g/(b-25+g))=0.25.

Assuming this inference is correct, we can find the number of boys using a two equation,two unknowns approach.

Thank you!

Yes, it's correct but if you simplify it you'd still get the same equation: \(b-3g=25\). Thus you'd still have only one equation with two unknowns.

Re: child is randomly selected from Columbus elementary school
[#permalink]

Show Tags

10 Aug 2017, 18:33

fitzpratik wrote:

If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75

(2) There are 35 more boys than there are girls

Hi.. For getting a ratio here, you require all terms with variable. But here by first look, you have a term without variable so none can be sufficient individually but let's see

1) \(\frac{b-25}{b+g-25}=0.75=\frac{3}{4}...... 4b-100=3b+3g-75....b=25+3g\) Ratio can't be found Insufficient 2) b=g+35 Again ratio cannot be found Insuff

Combined.. You can find values of b and g and thus get ratio or PROBABILITY 35+g=3g+25.....2g=10...G=5 b=35+5=40.. Probability of picking boy is b/t=40/40+5=8/9.. Suff C
_________________

Re: If a child is randomly selected from Columbus elementary sch
[#permalink]

Show Tags

28 Nov 2018, 07:50

Top Contributor

LinaNY wrote:

If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75

(2) There are 35 more boys than there are girls

Target question:What is the probability that the child will be a boy? This is a good candidate for rephrasing the target question. Let G = # of girls in the school Let B = # of boys in the school So, G + B = total number of children in the school So, P(selected child is a boy) = B/(G + B) REPHRASED target question:What is the value of B/(G + B)?

The video posted below has tips on rephrasing the target question

Statement 1: If 25 boys are removed from the school, the probability of selecting a boy will be 0.75. So, the number of boys = B - 25, and the total number of children = G + (B - 25) We can write: (B - 25)/(G + B - 25) = 3/4 Since we have a linear equation with TWO variables, there's no way to solve this equation for B and G. So, statement 1 is NOT SUFFICIENT

If you're not convinced, consider these two CONFLICTING cases: Case a: B = 28 and G = 1. After 25 boys leave, there are 3 boys and 1 girl. So, P(boy) = 3/4 = 0.75, which satisfies statement 1. In this case, the answer to the REPHRASED target question is B/(G + B) = 28/(1 + 28) = 28/29 Case b: B = 31 and G = 2. After 25 boys leave, there are 6 boys and 2 girls. So, P(boy) = 6/8 = 0.75, which satisfies statement 1. In this case, the answer to the REPHRASED target question is B/(G + B) = 31/(2 + 31) = 31/33 Since we cannot answer the REPHRASED target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: There are 35 more boys than there are girls. There are several CONFLICTING cases that satisfy statement 2. Here are two: Case a: B = 36 and G = 1. In this case, the answer to the REPHRASED target question is B/(G + B) = 36/(1 + 36) = 36/37 Case b: B = 37 and G = 2. In this case, the answer to the REPHRASED target question is B/(G + B) = 37/(2 + 37) = 37/39 Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined From statement 1, we can write: (B - 25)/(G + B - 25) = 3/4 Cross multiply to get: 3(G + B - 25) = 4(B - 25) Expand: 3G + 3B - 75 = 4B - 100 Rearrange to get: 3G - B = - 25

From statement 2, we can write: B = G + 35

At this point, we have two different linear equations with two variables. So, we COULD solve the system for B and G, which means we COULD answer the REPHRASED target question with certainty. So, the combined statements are SUFFICIENT

Answer: C

RELATED VIDEO FROM OUR COURSE

_________________

Test confidently with gmatprepnow.com

gmatclubot

Re: If a child is randomly selected from Columbus elementary sch &nbs
[#permalink]
28 Nov 2018, 07:50