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excelingmat
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HI excelingmat,

Just to clarify - is the 'source' of this question calling it a 'GMAT question' or are you just practicing with it as part of your studies?

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i don't think it is a gmat style question.
look at the question stem, what does 3:1 stand for? this information is relevant somehow? i have re-read the question several times
what does it want?
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excelingmat
Half of the volume of the milk and water mixture of ratio 7:5 is converted into a mixture of ratio 3:1 by the substitution(or replacement method) the mixture of ratio 7:5 was formed from the mixture 7:3 by adding the water in it. if 240 L milk is required in the replacement method ,what it the total amount of water was added t6o prepare the mixture 7:5 ?

a) 100 L
b) 50 L
c) 400 L
d) 300 L
e) 200 L

Source - NCERT


Hi,
ofcourse, this Q is not a typical GMAT Q and also not properly replicated from source..
but still lets solve it..
2 actions have been done on the mix...
1)7:3 is converted into 7:5 mix..
2) half of the mix of 7:5 is converted in 3:1 by adding/replacement of 240l milk..

lets start with action (2) as more info is given there..
let the mix common ratio be 2x..
so total sol = milk:water=7*2x:5*2x
240 l milk is added to half of it to make it 3:1..
so (7x+240)/5x =3:1..
we get x=30 or 2x=60..
so the sol is 7*60:5*60=420:300..

now we look at the first action ..
this was created from 7:3 mix...
so say x was added to get this ratio...
so 420/(300-x)=7/3..
here x =120..
but 120 does not seem to be a choice..

now, let us do by replacement but that only makes the question complicated and unlike a GMAT question.
Initial Ratio- 7*2x:5*2x
240 litre milk is added while 240 l of mix is removed. In this 240 l of mix we remove 240*7/12 or 140 l of milk.
Thus, the final product has 240-140 or 100 litre milk added to it instead of 100 l of water.

(7x+100)/(5x-100)=3/1
7x+100=15x-300 or 8x=400 or 2x=100
So initial quantity = 7*100:5*100 = 700:500

Let us say that y is removed in initial quantity to get it to 7:3 ratio
Thus, 700/(500-y)=7/3 or 2100=3500-7y or y=200
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excelingmat
Half of the volume of the milk and water mixture of ratio 7:5 is converted into a mixture of ratio 3:1 by the substitution(or replacement method) the mixture of ratio 7:5 was formed from the mixture 7:3 by adding the water in it. if 240 L milk is required in the replacement method ,what it the total amount of water was added t6o prepare the mixture 7:5 ?

a) 100 L
b) 50 L
c) 400 L
d) 300 L
e) 200 L

Source - NCERT

+1 for 120 as the answer..if the question is interpreted as is interpreted by chetan2u; other than that, worst wording.
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Bunuel

This question should be removed because it unnecessarily consumes limited available time of GMAT students which does not appear to belong in GMAT syllabus. Also, the expert solution of this ques by chetan2u does not match with the answer choices.
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Here is another way of solving this Problem..

Hope it helps..

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