Excellent quiz. However, the 5th question has another answer, 2x the one given. Perhaps you should specify that if more than 2 numbers fit the criteria, the smallest one is the one you want.
OK, I have changed it now so that there is only one answer. I had intended for there only to be one answer to each question. I have checked the rest and I think that is the only mistake. Please tell me if there are any more.