Suppose you have two people take an exam which is composed of 30 questions which are randomly chosen from a test bank of n questions.
Person A and Person B both take different randomly generated instances of the exam, and then compare the question sets they were given. Person B notices that 7 of their 30 questions were repeated from Person A's question set.
Is there anyway to deduce the likely total number of questions in the test bank given you know 7/30 of them were repeated in a second instance of the exam? Obviously you would not get an exact value, but could you determine a range of probabilities for each different size of the test bank? How you would you go about solving this?