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

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VP
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of the students at a certain school, 50% of the students are [#permalink] New post 15 Mar 2005, 07:13
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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 ?
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 [#permalink] New post 15 Mar 2005, 10:59
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 :-D
Maybe the moderators can help me with the attachement.
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 [#permalink] New post 15 Mar 2005, 11:09
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
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 [#permalink] New post 15 Mar 2005, 11:14
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 :-D
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 ? :-D

In any case, my working is the same as amit_upasan's.
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 [#permalink] New post 15 Mar 2005, 11:40
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
answer=3/20
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 [#permalink] New post 16 Mar 2005, 07:38
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
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 [#permalink] New post 16 Mar 2005, 20:49
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
  [#permalink] 16 Mar 2005, 20:49
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