1. The problem statement, all variables and given/known data Information in condensed form: => Student takes 20 questions true or false exam. => He knows answer to 10 questions. => He guesses on 10 questions. => Professor randomly picks two questions out of 20, and finds student answered them both correctly. Task: Find probability student knew answer to at least one of the two questions. 2. Relevant equations Bayes' Rule and conditional probability definition. 3. The attempt at a solution My work. I found P(K|C) = 2/3 via Bayes' rule, where K: student knows answer, C: he got it correct. However I'm not sure how to tackle "at least 1" part of problem. My thoughts were pick c1, c2 element of C and find P(K|(c1 or c2)), but this seems like a difficult calculation.