"What are you going to add?"
Translation:¿Qué van a añadir ustedes?
In this case, you need to place the tú after the verb to emphasise i.e. "Que vas a añadir tú?". You could also say "Tú, que vas a añadir?" (Notice you need the comma, as this sentence means something like "Hey you, what are you going to add?").
I'm sorry, I cannot really come up with the grammatical reasons for this right now...
cual and que both mean "what" but cual is more of a "which" kind of what.
If there had been a sentence before saying "Tienes tomates, papas, y pasta.", Cual vas a añadar would fit perfectly.
You have tomatoes, potatoes, and pasta. Which are you going to add?
however, in this example que is used because of the more generalized inquiry about what will be added.
I'm confused by this too. I read through this explanation and then chose cuál, so I'm clearly missing something! http://spanish.about.com/cs/grammar/a/que_vs_cual.htm
The difference is that the other sentence had an indirect object (the soup), whereas this sentence does not. In Spanish, you always have to include the indirect object pronoun (the "le"), even if the indirect object ("la sopa") is written out.
Now in this sentence, there is no indirect object. It's just "What are you going to add?"
However, if the sentence had been "What are you going to add to it," then you would need the "le," because "it" would be the indirect object.
So "Qué le vas a añadir?" = What are you going to add to it?
Because 'ustedes' is the subject. It's at the end because of inversion due to the question. The sentence doesn't ask "What are they/you going to add to you?" which is what it would say (kind of, because it's grammatically incorrect for not having the IOP 'les') with "a ustedes".