What is the probability that the student knew the answer to at least one of the two questions? A student takes a true-false examination containing 20
questions. On looking at the examination the student and that he
knows the answer to 10 of the questions which he proceeds to answer
correctly. He then randomly answers the remaining 10 questions. The
instructor selects 2 of the questions at random and  that the students answered both questions correctly.
What is the probability that the student knew the answer to at least one of the two questions?

Hint: What's the probability that the student guessed both correct questions randomly?
Hint 2: For that matter, you also need to model what happens when only one question is chosen by the instructor. What's the probability that the student chose randomly then? Remember that not every case has exactly 15 questions being answered correctly.
