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In a certain senior class, 72 percent of the male students [#permalink]
15 Aug 2009, 19:13

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Difficulty:

45% (medium)

Question Stats:

49% (01:49) correct
51% (01:13) wrong based on 136 sessions

In a certain senior class, 72 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

Re: What fraction of the students in the senior class are male? [#permalink]
15 Aug 2009, 20:21

3

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Expert's post

B

First of all, we have a red flag in the problem: it is a %-problem and it does not tell us about any absolute value, only about % and fraction. So, if we have one DS-statement only with one absolute value, the statement is useless for solving the problem.

1) Yeah, it stands here to catch us.

2) How to determine sufficiency of the statement for a few seconds? Just try slightly to increase or decrease 75% in your mind, do you need to change a fraction? Of course, as you need to have more/less male/female ratio to decrease/increase 75%. If it is difficult, the traditional solution:

Re: What fraction of the students in the senior class are male? [#permalink]
12 Mar 2011, 22:55

I got this problem wrong as usual...I HATE DS!

Anyway I checked the answer and it dawned on me that B is sufficient. However, I still cant seem to get 5/8, what am I doing wrong? X = m + f where X is total number of students in college. So from B) 75% of all students applied to college. Therefore we have 75% (X) Now we know 72% male students applied to college, therefore the male number will be 72% of 75% of X The fraction should be: ( 72% of 75% of X) / X This gives me around 54/100 males attending college.

Re: What fraction of the students in the senior class are male? [#permalink]
13 Mar 2011, 01:56

6

This post received KUDOS

Expert's post

thesfactor wrote:

I got this problem wrong as usual...I HATE DS!

Anyway I checked the answer and it dawned on me that B is sufficient. However, I still cant seem to get 5/8, what am I doing wrong? X = m + f where X is total number of students in college. So from B) 75% of all students applied to college. Therefore we have 75% (X) Now we know 72% male students applied to college, therefore the male number will be 72% of 75% of X The fraction should be: ( 72% of 75% of X) / X This gives me around 54/100 males attending college.

Check this: In a certain senior class, 72 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?

This is weighted average question.

Q: \frac{m}{m+f}=?

(1) There are 840 students in the senior class --> m+f=840. Stem info is useless. Not sufficient.

(2) 75 percent of the students in the senior class have applied to college --> 0.72m+0.8f=0.75(m+f) --> 3m=5f --> \frac{m}{f}=\frac{5}{3} --> \frac{m}{m+f}=\frac{5}{8}. Sufficient.

Re: What fraction of the students in the senior class are male? [#permalink]
31 May 2013, 14:38

walker wrote:

B

First of all, we have a red flag in the problem: it is a %-problem and it does not tell us about any absolute value, only about % and fraction. So, if we have one DS-statement only with one absolute value, the statement is useless for solving the problem.

1) Yeah, it stands here to catch us.

2) How to determine sufficiency of the statement for a few seconds? Just try slightly to increase or decrease 75% in your mind, do you need to change a fraction? Of course, as you need to have more/less male/female ratio to decrease/increase 75%. If it is difficult, the traditional solution:

75% = (72%*male + 80%*female)/(male + female) = (72% male/female+ 80%) / (male/female + 1) - we can fine male/female fraction.

Now, male/(male + female) = 1 / (1 + female/male) - using information from the statement above, we can find the fraction of the students who are male.

I was confused by your "quick" solution to determine sufficiency. Can you elaborate for me please?

Re: What fraction of the students in the senior class are male? [#permalink]
01 Jun 2013, 00:24

tmipanthers wrote:

walker wrote:

B

First of all, we have a red flag in the problem: it is a %-problem and it does not tell us about any absolute value, only about % and fraction. So, if we have one DS-statement only with one absolute value, the statement is useless for solving the problem.

1) Yeah, it stands here to catch us.

2) How to determine sufficiency of the statement for a few seconds? Just try slightly to increase or decrease 75% in your mind, do you need to change a fraction? Of course, as you need to have more/less male/female ratio to decrease/increase 75%. If it is difficult, the traditional solution:

75% = (72%*male + 80%*female)/(male + female) = (72% male/female+ 80%) / (male/female + 1) - we can fine male/female fraction.

Now, male/(male + female) = 1 / (1 + female/male) - using information from the statement above, we can find the fraction of the students who are male.

I was confused by your "quick" solution to determine sufficiency. Can you elaborate for me please?

Look closely at the alligation rule here, http://www.youtube.com/watch?v=SDA9buSrdlY So, if we know the avg of population 1 and avg of population 2 and also we know the combined avg of population 1 and 2 we can tell the proportion of population 1 and population 2 in combined population.

Male 72% and 80% female. Total 75% of class (class made of Male and Female. Hence total population) B gives the combined avg. Enough. No need to solve even.

Re: In a certain senior class, 72 percent of the male students [#permalink]
13 Oct 2013, 05:17

tejal777 wrote:

In a certain senior class, 72 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

This is a classic weighted average problem. Once you start to even think about drawing a matrix for an OS problem, check the statements out. You are given 2 data points and are asked for the ratio. If you have the weighted average you can know the ratio. The weighted average would just be in this case the TOTAL % that applied to college. Remember that for these questions we DON'T need the concrete value (Statement 1). All we need is the weighted average. Questions such as this one are pretty common, try to get used to spotting them right away.