Assume initial solution to be 40 L
Then 40L solution is added to 10% which is 4 L of water . important thing to note that since 10% of solution is removed after mixing , concentration will remain same after removing 10% of solution.
Just to be in the safe side for easy calculation, assume 10% of solution is not removed then the final solution becomes 44 L where milk and water will be in the ratio of 3:1. Since after removing 10% of this solution , ratio will still remain same , we can find out easily the initial ratio
So 3 parts of milk in final solution is before removing = (3/4)*44 = 33 L
and 1 part of water = 44-33 = 11L
Now water we have added was 4 L . So initial volume of water must be 11-4 = 7 L as volume is not changing before removal.
So the milk in initial solution will be 40-7= 33 L
Thus ratio should be initial solution irrespective of removal of solution at later stage = 33: 7. hence answer is C
_________________
You say this is a problem , I say this must be an opportunity