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