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# of the students at a certain school, 50% of the students are

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VP
Joined: 30 Sep 2004
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Location: Germany
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of the students at a certain school, 50% of the students are [#permalink]

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15 Mar 2005, 08:13
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of the students at a certain school, 50% of the students are in class a, 30% of the students are in class b, 20% of the students are in class c. of the students in class a, 20% are wearing eyeglasses. of the students in class b, 10% are wearing eyeglasses. of the students c, 10% are wearing eyeglasses. if a student is selected at random, what is the probability that the student is wearing eyeglasses ?
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5045
Location: Singapore
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Kudos [?]: 376 [0], given: 0

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15 Mar 2005, 11:59
I got 9/50.

My solution is in a word document as I used a table to solve the question. I can't display the table properly here. However, I also can't find a button to let me make an attachement. I don't know if I got my rights to attach files disabled or I am pure blind and can't find it
Maybe the moderators can help me with the attachement.
Intern
Joined: 17 Feb 2005
Posts: 28
Location: SE Michigan
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Kudos [?]: 39 [0], given: 0

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15 Mar 2005, 12:09
A:B:C
50:30:20

In A
Glasses: No Glasses
10:40

In B
Glasses: No Glasses
3:27

In C
Glasses: No Glasses
2:18

So in total,

15 wear glasses out of 100

req. probability= 15/100
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5045
Location: Singapore
Followers: 31

Kudos [?]: 376 [0], given: 0

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15 Mar 2005, 12:14
ywilfred wrote:
I got 9/50.

My solution is in a word document as I used a table to solve the question. I can't display the table properly here. However, I also can't find a button to let me make an attachement. I don't know if I got my rights to attach files disabled or I am pure blind and can't find it
Maybe the moderators can help me with the attachement.

Made a mistake with my table. Yes, answer is 3/20 (15/100). But I'm still not able to attach my document. Can someone tell me why ?

In any case, my working is the same as amit_upasan's.
Manager
Joined: 15 Feb 2005
Posts: 246
Location: Rockville
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15 Mar 2005, 12:40
A:(prob of picking a student from class A)*(prob of picking a student with glasses)=2/10*1/2
do the same for a student from class B and C
B: 3/10*1/10
C: 2/10*1/10
A+B+C
VP
Joined: 30 Sep 2004
Posts: 1482
Location: Germany
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Kudos [?]: 347 [0], given: 0

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16 Mar 2005, 08:38
0,5*0,2+0,3*0,1+0,2*0,1=0,15=15%

now it is easy to see the connection to the other question => http://www.gmatclub.com/phpbb/viewtopic.php?t=14826
Manager
Joined: 17 Dec 2004
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16 Mar 2005, 21:49
I got 3/20 as well:

A - 50% students, 20% wear glasses = 10% of total students
B - 30% students, 10% wear glasses = 3% of total students
C - 20% students, 10% wear glasses = 2% of total students

which leads to 15% of total students wear glasses

15/100 = 3/20
16 Mar 2005, 21:49
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# of the students at a certain school, 50% of the students are

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