GMAT Review PS #188 Why didn't Plugging In work?

Greetings all. Here is the problem I have a question on, from the GMAT review 11th edition: Drum X is 1/2 full of oil and drum Y, which has twice the capacity of drum X, is 2/3 full of oil. If all of the oil in drum X is poured into drum Y, then drum Y will be filled to what fraction of its capacity...