You can sometimes get away with admitting ignorance if you promise to follow up with action. As in "I don't know the exact particulars of Y, but I'll look into it later today and get back to you tonight." This is one of the things that students getting their first job who have spent all their lives in school don't get. They are so used to the academic environment and its expectations of always knowing on the spot, that they don't realize it's sometimes permissible not to know -- you just have to state it appropriately.