Ustedes means "you," when referring to a group of people and using formal voice. It doesn't have to have "all" with it to mean plural, right? If I was a teacher, addressing three kids, I could say "you cheated on the test, so you fail. I wouldn't have to say "you all cheated so you all fail" if it was obvious that I was speaking to the group.
ustedes is a term for a plural amount of people, so technically it does translate you all, however, like in English, most people do not say you all unless it is necessary. i believe it is to be interpreted as either a plural you or a singular you and since we aren't given enough information to know if you cook is being said to one or more other people, we should assume that they want all possible solutions.