the mixture of milk and water is in the ratio of 3:1 how much mixture must be withdraw and water replaced in order to get resulting mixture half milk and half water

Suppose initially there is 4 liters of mixture.

Let x liters of this liquid be replaced with water.

Quantity of water in new mixture = 1 - x/4 + x

Quantity of milk in new mixture = 3 - 3x/4

 

According to the given information:

1 - x/4 + x = 3 - 3x/4

- x/4 + x + 3x/4 = 3 - 1

x = 4/3

 

Thus, part of the mixture replaced = 4/3 x 1/4 = 1/3