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3. a) [3 points] An urn contains 7 red balls and 6 blue balls. A sample of 5 balls is selected. How many different samples are possible ? How many samples contain exactly 3 red balls ? How many samples contain no red balls ? b) [2 points] In how many ways is it possible to sit seven knights at a round table if Friday, Saturday and Sunday insist on sitting together ? In how many ways is it possible to sit them if Wednesday refuses to sit next to Saturday or Sunday