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ratio and proportion

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by DJHJA » Mon Dec 13, 2010 9:49 pm
In a class, 10% of the girls have blue eyes, and 20% of the boys have blue eyes.
If the ratio of girls to boys in the class is 3:4, then what is the fraction of students in the class having blues eyes?


the answer is 11/70, however the solution given is;

let 1 represent the total # of students
1-(.1+.2)= .7 ( why is this done??? :? why isnt it .1+.2=.3 as the denominator)

girls(with blue eyes) + boys(with blue eyes) =
(3*.1 )+(4*.2)/ .7 = .3+.8/.7= .11/.7 = 11/70

why is .7 the denominator and not .3.
isnt .3 the total percentage of students with blue eyes?
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by Night reader » Mon Dec 13, 2010 10:07 pm
DJHJA wrote:In a class, 10% of the girls have blue eyes, and 20% of the boys have blue eyes.
If the ratio of girls to boys in the class is 3:4, then what is the fraction of students in the class having blues eyes?


the answer is 11/70, however the solution given is;

let 1 represent the total # of students
1-(.1+.2)= .7 ( why is this done??? :? why isnt it .1+.2=.3 as the denominator)

girls(with blue eyes) + boys(with blue eyes) =
(3*.1 )+(4*.2)/ .7 = .3+.8/.7= .11/.7 = 11/70

why is .7 the denominator and not .3.
isnt .3 the total percentage of students with blue eyes?
girls/boys=3/4 => girls/total=3/7 & boys/total=4/7 <=> 3/7 + 4/7 = 1 (girls+boys over total)

blue eyed boys 20% or 1/5
blue eyed girls 10% or 1/10

how many of all boys are blue eyed => 1/5 * 4/7 = 4/35
how many of all girls are blue eyed => 1/10 * 3/7 = 3/70

how many of all boys and girls are blue eyed => 4/35 + 3/70 = 11/70

many OA explanations may differ in notation and decimal translations; we need to know the method only
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by DJHJA » Mon Dec 13, 2010 10:23 pm
omg thanks ur a life saver
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by frank1 » Tue Dec 14, 2010 1:00 am
When you see two set of mutually exclusive sets use this method...
it saves time
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11/70 clear from it
GMAT score is equally counted as your GPA and 78 clicks can change you life.
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by ikaplan » Tue Dec 14, 2010 7:22 am
Double matrix method is one of the most useful techniques I have learned during the GMAT prep period. Frank1, thanks for reminding us to use it!
"Commitment is more than just wishing for the right conditions. Commitment is working with what you have."
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