There is a logical argument I'd like to have some comments on. I read it in a book, and I am not sure about it. "An instructor teaches a class five days a week, Monday through Friday. She tells her class that she will give one more quiz during the final week of classes, but that the students will not know for sure the quiz will be that day until they come to the classroom. What is the last day of the week she can give toe quiz to satisfy these conditions?" Consider the following proof: The teacher cannot give the students their tests on Friday, because then they would know on Thursday (not being tested) when they will get their test. Thus Friday is eliminated, and Thursday is the last possible day of being tested. However, if they did not receive their test on Wednesday, they would know what day they would receive the test - again violating the rules. Thus Thursday is eliminated. The argument continues to eliminate Wednesday and Tuesday, and thus Monday is the only possible day left to receive the test. This also violates the rules, since they would know it beforehand. I do have some comments on this argument. For example, when eliminating Friday, we assume Friday already is a possible day for receiving the test. However, when eliminating Thursday we use the conclusion while simultaneously considering Friday impossible. Would you consider this a dismantle of the previous proof? The proof does imply that the students know the test will be given Monday. But if the teacher decide to give it on Wednesday, this knowledge is false. However, they also know that the teacher will not violate the rules: so is giving the test at all a violation of the rules? I'd like some comments on the possible proof, and the possible dismantling of the proof. And finally, do you know the correct answer?