I don't know if it's really a bad thing for students to assume that their teachers are not going to mess with them by giving them an unsolveable problem, doesn't that just indicate that they trust their teacher? In the case of real world problems, there may or not be a specific correct answer, and I think students know that on some level. Maybe a better way to ask that question would be, "Suppose you know that there are 125 sheep and 5 dogs in a flock. Your friend Amelia asks you, 'How old is the shepherd?' What would be your response?" That way the hypothetical nature of the question is more obvious.