If you drink half this time, it is half of half milk, plus half of water.

That is 1/4 milk and 1/4 water.

For the third time, there should be 1/4 milk and 1/4 water left (1/2-1/4 =1/4). After the water is filled, pour 1/2 water.

Milk:1/2+1/4+1/4 =1(cup)

Water:1/2+1/4+1/4 =1(cup)