I could not find an answer for this question but I think the answer is B) -- Can anyone please tell me B) is correct?
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15. A certain used-book dealer sells paperback books at 3 times dealer's cost and hardback books at 4 times dealer's cost. Last week the dealer sold a total of 120 books, each of which had cost the dealer $1. If the gross profit (sales revenue minus dealer's cost) on the sale of all of these books was $300, how many of the books sold were paperbacks?
(A) 40
(B) 60
(C) 75
(D) 90
(E) 100