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 180 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?	Options:
A) 100
B) 110
C) 120
D) 130
E) 140
The dealer earns a profit of $2 per paperback and $3 per hardback.   If all 180 books were paperbacks, the dealer would earn a profit of $360.   Since the actual profit was $400, the dealer must have sold 40 hardbacks.   Then the number of paperbacks sold was 180-40= 
<gadget id="calculator">180-40</gadget>
<output>140</output>
140.   The answer is 140.
<result>E</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 180 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?