for part iv, where does the 4c1 and 5c1 come from? the question need 3 marbles , but there's only 2 marbles are chosen based on the working. please correct me if i am wrong.
http://ift.tt/1oEC1oh
http://ift.tt/1oEBYZJ
http://ift.tt/1oEC1oh
http://ift.tt/1oEBYZJ
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