if you don't know the answer to a particular question, simply admit that you don't know. say something like this:
"i'm sorry. i don't happen to know the answer to that question, but i'll be happy to check into it for you."
here are some further examples:
i don't think i can answer your question.
i wish i could answer your question, but unfortunately i have no good answer.
i'm not sure that i can answer your question. what i'm going to say is not quite an answer to your question.
another technique to cope with the situation is to direct the attention to another expert who may know it or you can use the audience. ask if anyone could help the questioner and hence it won't be necessary for you to admit that you do not know the answer. see examples below.