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OpenStudy (anonymous):
Please help me with this probability question :) 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?
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