3 identical containers are completely filled with water-milk mixtures in the ratio 2:3, 3:4 and 4:5 respectively. All these mixtures are poured into a bigger container. What fraction of mixture in the bigger container should be replaced by water so that the resulting mixture has equal quantities of water and milk?

Let the container volume be 315 units.
So,
1st container : 126unit : 189unit
2nd container : 135unit : 180unit
3rd container : 140unit : 175unit

Quantity in bigger container : 401unit : 544unit

Let the amount taken out be 'x' units.
Then ,
401+x-(401/945)x = 544-(544/945)x
401+(544/945)x = 544-(544/945)x
1088x/945 = 143
x = 143*945/1088
x = 124.2

fraction required = 124.2/945 = 13/100 approx.

Thanks a lot!