Four cups of milk are to be poured into a 2-cup bottle and

I am a big fan of trying to solve most GMAT questions orally.
Look at it this way:
I have four cups of milk. I also have a 2 cup bottle and a 4 cup bottle. When will they have the same fraction of capacity? Since 4 cup bottle's capacity if twice of 2 cup bottle's capacity, they will have the same fraction when 4 cup bottle has twice the milk contained in 2 cup bottle. So we just have to divide the 4 cups of milk in the ratio 1:2 and give two parts to the 4 cup bottle.
So (2/3)* 4 = 8/3


Note: If they had given that the milk to be poured is 3 cups, you would have automatically said that we should put 1 cup and 2 cups in 2 cup bottle and 4 cup bottle respectively. They give such numbers to make the question a little harder.