Explanation:

Here total parts of milk and water in the solution is 6+2 = 8 parts.

1part = 640/8 = 80

old mixture contains 6parts of milk and 2 parts of water(6:2).

To get new mixture containing half milk and half water,

add 4parts of water to the old mixture

then 6:(2+4) to make the ratio same.

=4 x 80 = 320 ml.

Hence, option 4 is the correct answer.