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# In a certain senior class, 75 percent of the male students

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Senior Manager
Joined: 22 Jun 2005
Posts: 363
Location: London
In a certain senior class, 75 percent of the male students [#permalink]

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08 Apr 2006, 09:50
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In a certain senior class, 75 percent of the male students and 80 percent of the female students have applied to college. What fraction of the students in the senior class are male?

1. There are 840 students in the senior class
2. 75 percent of the students in the senior class have applied to college
Manager
Joined: 25 Mar 2006
Posts: 129
Location: Stonybrook Univ

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08 Apr 2006, 11:00
statement 1 ... nothing much here

statement 2 gives us .75x(where x is the number of males)+.80y(where y is the number of females)=.75 z( z is x+y or total number of students )

.75x+.80y=.75z we can write y as y=z-x then we get

.75x+.80z-.80x=.75z ... we need to solve for (x/z) and this could be done

but this only gives us the fraction of the males which applied to college ... we still dont know about the ones that didnt..

am i missing something ?
Manager
Joined: 09 Feb 2006
Posts: 129
Location: New York, NY

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08 Apr 2006, 13:50
I just got this same question in GMATPrep. I put E. We need to know the percentage of students that are male to answer this question (at least that's what I thought), but we don't. The OA is B, but that made no sense to me.
Senior Manager
Joined: 22 Nov 2005
Posts: 474

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08 Apr 2006, 14:13
jcgoodchild wrote:
I just got this same question in GMATPrep. I put E. We need to know the percentage of students that are male to answer this question (at least that's what I thought), but we don't. The OA is B, but that made no sense to me.

jcgoodchild,

trick here is its asking ratio of number male students to total number of students. So we don't need A because we can replace female students by total students - Male students.

Hope this will help.
Manager
Joined: 25 Mar 2006
Posts: 129
Location: Stonybrook Univ

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08 Apr 2006, 14:34
gmat_crack can you please solve this question fully.
Manager
Joined: 09 Feb 2006
Posts: 129
Location: New York, NY

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08 Apr 2006, 18:08
GMAT_Crack -- I realized what the question is asking. I had written out a whole post explaining the issue when I think I stumbled on the answer.

Ok, so we want to find the percentage of students who are male. We know a couple of things. 72% of males applied to college and 80% of females applied to college, and from B we know that this is equal to 75% of the student body.

(.72m + .8f) / (m+f) = .75

To your point, they want the ratio of male to femal students.

.72m + .8f = .75m + .75f
.05f = .03m

m/f = .05/.03 = 5/3

So the fraction of male students is 5/8 ths. Sound right?
VP
Joined: 28 Mar 2006
Posts: 1369

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08 Apr 2006, 18:15
m+f = t (total) ----(i)

.75m + .8f = .75t ----(ii)

Divide (i) by z

m/t + f/t = 1 ------(iii)

divide (ii) also by "t" you get the value of f/t = 1- m/t

So B is enough 'cos you are asked for the portion of the males in the total class but not the # os males....
Manager
Joined: 21 Dec 2005
Posts: 102

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08 Apr 2006, 18:22
jcgoodchild wrote:
GMAT_Crack -- I realized what the question is asking. I had written out a whole post explaining the issue when I think I stumbled on the answer.

Ok, so we want to find the percentage of students who are male. We know a couple of things. 72% of males applied to college and 80% of females applied to college, and from B we know that this is equal to 75% of the student body.

(.72m + .8f) / (m+f) = .75

To your point, they want the ratio of male to femal students.

.72m + .8f = .75m + .75f
.05f = .03m

m/f = .05/.03 = 5/3

So the fraction of male students is 5/8 ths. Sound right?

If you use .75 as given in the question instead of .72 you won't get a ratio!!
SVP
Joined: 14 Dec 2004
Posts: 1689

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08 Apr 2006, 21:13
IMO, what we need is % of male students in the senior class. (fraction)

Let's assume there are 100 students in senior class.
Then,
0.75x + 0.8(100-x) = 0.75

We can solve for "x", which in itself is a fraction (or percent), So "B".
Senior Manager
Joined: 22 Nov 2005
Posts: 474

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08 Apr 2006, 21:30
jcgoodchild wrote:
GMAT_Crack -- I realized what the question is asking. I had written out a whole post explaining the issue when I think I stumbled on the answer.

Ok, so we want to find the percentage of students who are male. We know a couple of things. 72% of males applied to college and 80% of females applied to college, and from B we know that this is equal to 75% of the student body.

(.72m + .8f) / (m+f) = .75

To your point, they want the ratio of male to femal students.

.72m + .8f = .75m + .75f
.05f = .03m

m/f = .05/.03 = 5/3

So the fraction of male students is 5/8 ths. Sound right?

Hey dude,
You are calculating the ratio of m/f, but we need here m/T where T is the total number of students.
so T = m + f
--> f = T-m
Now in equation in blue, replace f = t-m

------> [ .72m + .8(T-m)] / T = .75

from this we need to get m/T.

---------> .8 - .08*m/T = .75

---------> .05 = .08*m/T
---> m/T = 5/8

Hope this will help everyone.
Manager
Joined: 21 Dec 2005
Posts: 102

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08 Apr 2006, 22:06
vivek123 wrote:
IMO, what we need is % of male students in the senior class. (fraction)

Let's assume there are 100 students in senior class.
Then,
0.75x + 0.8(100-x) = 0.75

We can solve for "x", which in itself is a fraction (or percent), So "B".

If you solve for x you get a value that is greater than 100 students which is not correct.
Manager
Joined: 21 Dec 2005
Posts: 102

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08 Apr 2006, 22:07
gmat_crack wrote:
jcgoodchild wrote:
GMAT_Crack -- I realized what the question is asking. I had written out a whole post explaining the issue when I think I stumbled on the answer.

Ok, so we want to find the percentage of students who are male. We know a couple of things. 72% of males applied to college and 80% of females applied to college, and from B we know that this is equal to 75% of the student body.

(.72m + .8f) / (m+f) = .75

To your point, they want the ratio of male to femal students.

.72m + .8f = .75m + .75f
.05f = .03m

m/f = .05/.03 = 5/3

So the fraction of male students is 5/8 ths. Sound right?

Hey dude,
You are calculating the ratio of m/f, but we need here m/T where T is the total number of students.
so T = m + f
--> f = T-m
Now in equation in blue, replace f = t-m

------> [ .72m + .8(T-m)] / T = .75

from this we need to get m/T.

---------> .8 - .08*m/T = .75

---------> .05 = .08*m/T
---> m/T = 5/8

Hope this will help everyone.

It is not .72 it is .75 and you cannot deduce the fraction if you follow that equation

Last edited by jodeci on 08 Apr 2006, 22:16, edited 1 time in total.
SVP
Joined: 14 Dec 2004
Posts: 1689

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08 Apr 2006, 22:08
jodeci wrote:
vivek123 wrote:
IMO, what we need is % of male students in the senior class. (fraction)

Let's assume there are 100 students in senior class.
Then,
0.75x + 0.8(100-x) = 75

We can solve for "x", which in itself is a fraction (or percent), So "B".

If you solve for x you get a value that is greater than 100 students which is not correct.

oops, i made a mistake....corrected in red.
Manager
Joined: 21 Dec 2005
Posts: 102

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08 Apr 2006, 22:19
vivek123 wrote:
jodeci wrote:
vivek123 wrote:
IMO, what we need is % of male students in the senior class. (fraction)

Let's assume there are 100 students in senior class.
Then,
0.75x + 0.8(100-x) = 75

We can solve for "x", which in itself is a fraction (or percent), So "B".

If you solve for x you get a value that is greater than 100 students which is not correct.

oops, i made a mistake....corrected in red.

Still not right, you get 100 for x which is the same as the number you chose for all students meaning female students are zero.
Senior Manager
Joined: 05 Jan 2006
Posts: 381

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09 Apr 2006, 08:54
trivikram wrote:
m+f = t (total) ----(i)

.75m + .8f = .75t ----(ii)

Divide (i) by z

m/t + f/t = 1 ------(iii)

divide (ii) also by "t" you get the value of f/t = 1- m/t

So B is enough 'cos you are asked for the portion of the males in the total class but not the # os males....

Agree with analysis B is enough, though I ended up selecting E at first glance...
Intern
Joined: 01 Feb 2006
Posts: 34

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09 Apr 2006, 09:44
I keep getting m/t = 1, which makes little sense to me. It seems that everyone is getting this ratio too. Perhaps this is truly the answer. I've been scouring my calculation for mistakes but everything I have down is logical. I don't think there are any females in this school....

I've tried it through formulas and charts. Everything points to 0 females and all males. Hell, you can even use the 1st statement they give you and try it out with that. It'll give you 840 males.

Good luck...
VP
Joined: 29 Dec 2005
Posts: 1341

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09 Apr 2006, 14:04
macca wrote:
In a certain senior class, 75 percent of the male students and 80 percent of the female students have applied to college. What fraction of the students in the senior class are male?

1. There are 840 students in the senior class
2. 75 percent of the students in the senior class have applied to college

the question seems poorly structured for me because if there are male and female, both, students in the college, 75 percent of the male students and 80 percent of the female comprise more than 75% of total.

but if 75 percent of the male students and 80 percent of the female comprise more than 75% of total, then thre are only male students. so the fraction of male student is 1 (or 100%).

lets suppose total = 100
male = x
0.75x + 0.8(100-x) = 75
0.75x + 80 -0.8x = 75
0.75x -0.8x = 75-80
0.05x = 5
x = 100

if so, B is enough...
Senior Manager
Joined: 08 Sep 2004
Posts: 257
Location: New York City, USA

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10 Apr 2006, 08:45
Here is my take on this -

using the same convention, x - male, y - female, T - total

Condition1 : Doesn't give us anything.

Condition2 :
0.75 x + 0.80y = 0.75T - where T = x + y
=> 0.75x + (0.75 + 0.05) y = 0.75T
=> 0.75(x+y) + 0.05y = 0.75T
=> 0.75T + 0.05y = 0.75T

=> y = 0, i.e. there are no females in the class. So what fraction of the students are male, 100%.

Hence B.
Senior Manager
Joined: 22 Jun 2005
Posts: 363
Location: London

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11 Apr 2006, 01:50
yeah, but it's from gmatprep!!
Manager
Joined: 25 Mar 2006
Posts: 129
Location: Stonybrook Univ

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11 Apr 2006, 04:22
hehe this problem sucks the only reason i would think C is because i kept getting 0 females ... oh well hope i dont get something this silly on my test
11 Apr 2006, 04:22

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