Question

How many litres of fresh water should be mixed with 30 litres of 50% milk solution so that resultant solution is a 10% milk solution?
(a) 120 litres
(b) 25 litres
(c) 150 litres
(d) 60 litres
(e) 160 litres

Answer 1

One easy way of solving this problem has already been posted.
Here's another:
In current mixture there is 4L of water and 36L of milk.
We take x= water present in final solution.
Now 36+x=total volume of final solution
that means 20% of (36+x) = x
0.2(36 +x)=x
36+x=5x
4x=36
x=9
Since there was 4L of water already there, you need to add another 5 litres.

One more way:
x=total volume of final solution.
So water present in it will be x-36
Thus x-36=20% of x
x-36=0.2x
0.8x=36
x=45
Difference= 45-40=5L
Since we took the volume of milk as a fixed 36 litres, an extra 5 litres of water has to be added.