A student has an assignment with 6 questions. The professor has told him
that he has to do all the questions, but that only the 3 most important questions will
be marked. The student does not know which 3 questions are the important ones, but
he decides to take his chances and just do 3 of the question at random and hope that
those are the questions that the professor is going to mark. What are the chances that
the student does exactly the three question that the professor is going to grade?