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 140 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? Pick one
A) 20 B) 40 C) 60 D) 80 E) 100
The dealer earns a profit of $2 per paperback and $3 per hardback.   If all 140 books were paperbacks, the dealer would earn a profit of $280.   Since the actual profit was $400, the dealer must have sold 120 hardbacks.   Then the number of paperbacks sold was 140-120= 
<gadget id="calculator">140-120</gadget>
<output>20</output>
20.   The answer is 20.
<result>A</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 140 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?