A used-book dealer sells paperback books at 3 times dealer's cost and hardback books at 4 times the dealer's cost. Last week, the dealer sold a total of 160 books, each of which had cost the dealer $1. If the gross profit (sales revenue minus the dealer's cost) on the sale of all of these books was $400, how many of the books sold were paperbacks?	Choices:
A) 60 B) 80 C) 100 D) 120 E) 140
The dealer earns a profit of $2 per paperback and $3 per hardback.   If all 160 books were paperbacks, the dealer would earn a profit of $320.   Since the actual profit was $400, the dealer must have sold 80 hardbacks.   Then the number of paperbacks sold was 160-80= 
<gadget id="calculator">160-80</gadget>
<output>80</output>
80.   The answer is 80.
<result>B</result>
A used-book dealer sells paperback books at 3 times dealer's cost and hardback books at 4 times the dealer's cost. Last week, the dealer sold a total of 160 books, each of which had cost the dealer $1. If the gross profit (sales revenue minus the dealer's cost) on the sale of all of these books was $400, how many of the books sold were paperbacks?