Author 
Message 
TAGS:

Hide Tags

Intern
Joined: 03 Jul 2013
Posts: 27
Location: United States
Concentration: Finance, General Management
GPA: 3
WE: Human Resources (Consulting)

In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
22 Sep 2013, 13:57
Question Stats:
56% (01:21) correct 44% (01:40) wrong based on 299 sessions
HideShow timer Statistics
In a certain class, the ratio of girls to boys is 5:4. How many girls are there? (1) If four new boys joined the class, the number of boys would increase by 20%. (2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23
Official Answer and Stats are available only to registered users. Register/ Login.



Manager
Joined: 27 Feb 2012
Posts: 125

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
22 Sep 2013, 18:50
Dhairya275 wrote: In a certain class, the ratio of girls to boys is 5:4. How many girls are there?
(1) If four new boys joined the class, the number of boys would increase by 20%.
(2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23
Easy one ! but still not able to think any easy way out ! Help Please 1) G/B = 5/4. 4x + 4 = 1.2 * 4x You can find x and hence ans. 2) 8/23 = 4x/(9x +2.5x) x will cancel. So only A stands.
_________________

Please +1 KUDO if my post helps. Thank you.



Math Expert
Joined: 02 Sep 2009
Posts: 49320

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
22 Sep 2013, 23:37
Dhairya275 wrote: In a certain class, the ratio of girls to boys is 5:4. How many girls are there?
(1) If four new boys joined the class, the number of boys would increase by 20%.
(2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23
Easy one ! but still not able to think any easy way out ! Help Please In a certain class, the ratio of girls to boys is 5:4. How many girls are there?(1) If four new boys joined the class, the number of boys would increase by 20%. This implies that 4 boys constitute for 20% of the original #of boys. Thus there are 20 boys in the class > there are 25 girls in the class. Sufficient. (2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23: \(\frac{boys}{(girls*1.5+boys)}=\frac{8}{23}\); \(\frac{4x}{(5x*1.5+4x)}=\frac{8}{23}\); \(\frac{4x}{(11.5x)}=\frac{8}{23}\); x cancels: \(\frac{4}{11.5}=\frac{8}{23}\). Not sufficient. Answer: A.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Manager
Joined: 23 Jan 2013
Posts: 156
Concentration: Technology, Other
GMAT Date: 01142015
WE: Information Technology (Computer Software)

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
18 Mar 2014, 19:12
Given ratio of B/G = 5/4 .
Stmt 1 : B+ 4= 1.2 B ie B = 20 , can find out girls sufficient .
Stmt 2 : Probability boy would be selected after girls increase by 50 % = B / B + 1.5 G = 8/23 . ie B/G = 4/5 . Not sufficient .
Hence answer is A



Intern
Joined: 23 Mar 2014
Posts: 12

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
27 Apr 2014, 08:51
Could someone provide a more detailed explanation in regards to why Statement 2 is not sufficient? First off, I wouldn't know how to approach statement 2.
Thanks!



Math Expert
Joined: 02 Sep 2009
Posts: 49320

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
28 Apr 2014, 03:10
beef001 wrote: In a certain class, the ratio of girls to boys is 5:4. How many girls are there?
Statement (1): If four new boys joined the class, the number of boys would increase by 20%.
Statement (2): If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23
Could someone provide a more detailed explanation in regards to why Statement 2 is not sufficient? First off, I wouldn't know how to approach statement 2.
Thanks! The stem gives us the ratio: the ratio of girls to boys is 5:4. So, there are 5x girls and 4x boys, for some positive integer x, The second statement also gives a ratio but in a different way: if the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23. This is basically the same info as we had from the stem. If the number of girls increases from 5x to 7.5x, then the probability that a randomly chosen student would be a boy would be (boys)/(new total) = 4x/(7.5x+4x) = 4x/11.5x = 8/23. So, the second statement does not give us any info we did not know ourselves from the stem, which means that it's not sufficient. Does this make sense?
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Director
Joined: 21 May 2013
Posts: 648

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
09 Dec 2014, 01:23
Bunuel wrote: beef001 wrote: In a certain class, the ratio of girls to boys is 5:4. How many girls are there?
Statement (1): If four new boys joined the class, the number of boys would increase by 20%.
Statement (2): If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23
Could someone provide a more detailed explanation in regards to why Statement 2 is not sufficient? First off, I wouldn't know how to approach statement 2.
Thanks! The stem gives us the ratio: the ratio of girls to boys is 5:4. So, there are 5x girls and 4x boys, for some positive integer x, The second statement also gives a ratio but in a different way: if the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23. This is basically the same info as we had from the stem. If the number of girls increases from 5x to 7.5x, then the probability that a randomly chosen student would be a boy would be (boys)/(new total) = 4x/(7.5x+4x) = 4x/11.5x = 8/23. So, the second statement does not give us any info we did not know ourselves from the stem, which means that it's not sufficient. Does this make sense? Bunuel, Can we assume the ratio as 10x and 8x and then solve statement 2? That will be the only case where the given probability will hold true. Then the answer will be D.



Senior Manager
Joined: 13 Jun 2013
Posts: 277

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
09 Dec 2014, 03:01
KS15 wrote: Can we assume the ratio as 10x and 8x and then solve statement 2? That will be the only case where the given probability will hold true. Then the answer will be D. hi, to find the total number of girls, we must calculate the value of x. in this case also (10x and 8x), value of x gets cancel out when we calculate the probability of selecting a boy from the given set of students.



Intern
Joined: 10 Nov 2013
Posts: 18

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
02 Jul 2017, 14:55
(2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23 > boys/(girls*1.5+boys)=8/23 > 4x/(5x*1.5+4x)=8/23 > 4x/(11.5x)=8/23 > x cancels: 4/11.5=8/23. Not sufficient.
Answer: A.[/quote]
Doesn't 8/23 imply that there are 8 boys out of 23 total people? What am I missing?



Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 6227
GPA: 3.82

Re: In a certain class, the ratio of girls to boys is 5:4. How
[#permalink]
Show Tags
02 Jul 2017, 20:07
mikemcgarry wrote: In a certain class, the ratio of girls to boys is 5:4. How many girls are there?
Statement (1): If four new boys joined the class, the number of boys would increase by 20%.
Statement (2): If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23Ratios, proportions, and percents! Oh my! For a detailed discussion of ratios and their powerful problemsolving potential, as well as for the OE of this question, see: http://magoosh.com/gmat/2013/gmatquant ... oportions/Mike Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution. Let \(b\) and \(g\) be the numbers of boys and girls, respectively. \(b:g = 4:5\) or \(5b = 4g\). There are 2 variables and 1 equation. Thus D is the answer most likely. Condition 1) From the condition 1), we have \(4 = \frac{1}{5b}\) or \(b = 20\). Thus g = 25. This is sufficient. Condition 2) We have \(\frac{b}{{b + 1.5g }} = \frac{8}{23}\) or \(23b = 8b + 12g\). It is equivalent to \(15b = 12g\) or \(5b = 4g\) which is redundant since it is exactly same as the condition of the original question. Thus this is not sufficient. Therefore the answer is A. > For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The oneandonly World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only $99 for 3 month Online Course" "Free Resources30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons  try it yourself"



Math Expert
Joined: 02 Sep 2009
Posts: 49320

In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
03 Jul 2017, 02:49



Intern
Joined: 09 Apr 2018
Posts: 2

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
09 Apr 2018, 21:34
For Statement (2) 
If you set it up as 1.5(5x) = 15/23, you can get a unique value for x and therefore a value for 5x (which is the ratio of girls). Why is that not sufficient?
Thanks very much in advance



Math Expert
Joined: 02 Sep 2009
Posts: 49320

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
09 Apr 2018, 21:41



Intern
Joined: 09 Apr 2018
Posts: 2

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
11 Apr 2018, 08:26
Bunuel  thanks for the response. My logic is as follows. Start with the following statement: (2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23 If this is the case, then given that the original proportion of Girls to Boys is 5x : 4x, then girls have increased to 1.5*(5x). After this increase, the total probability that is would be a girl is 1  8/23, or 15/23. As a result, I can set up the following equation below and solve for x, which will solve for the total number of girls? 1.5*(5x) = 15/23



Math Expert
Joined: 02 Sep 2009
Posts: 49320

Re: In a certain class, the ratio of girls to boys is 5:4. How many girls
[#permalink]
Show Tags
11 Apr 2018, 08:32
KNA32 wrote: Bunuel  thanks for the response. My logic is as follows. Start with the following statement: (2) If the number of girls increases by 50%, then after such an increase, the probability that a randomly chosen student would be a boy would be 8/23 If this is the case, then given that the original proportion of Girls to Boys is 5x : 4x, then girls have increased to 1.5*(5x). After this increase, the total probability that is would be a girl is 1  8/23, or 15/23. As a result, I can set up the following equation below and solve for x, which will solve for the total number of girls? 1.5*(5x) = 15/23 You are making the same mistake... 15/23 as well as 8/23 represent probability, which is a ratio. The left hand side should be (girls)/(total) = 1.5*(5x)/(1.5*(5x) + 4x)
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics




Re: In a certain class, the ratio of girls to boys is 5:4. How many girls &nbs
[#permalink]
11 Apr 2018, 08:32






