Mark has a bag that contains 3 black marbles, 6 gold marbles, 2 purple marbles, and 6 red marbles. Mark adds a number of white marbles to the bag and tells Susan that if she now draws a marble at random from the bag, the probability of it being black or gold is 73. The number of white marbles that Mark adds to the bag is
(A) 5 (B) 2 (C) 6 (D) 4 (E) 3
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Source: CEMC, University of Waterloo,
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Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.