Here is the answer explanation.
The resultant concentration when the three solutions are mixed as given, will be
KX+2KY+3K(X+Y) / K+2K+3K = 4X+5Y/6 = 50% (Given)
Now, in the second case the resultant concentration will be 40x+50y/90 = 4x+5y/9
However, since 4x+5y/6 = 50% ,
4x+5y/9 = 50% * 2/3 = 33.33%
So Answer is B.
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