问题
单项选择题
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 paper backs ？（）
A．40
B．60
C．75
D．90
E．100
答案
参考答案：B
解析：
设该书商买了x本平装本的书，买了y本精装本的书，则根据题意可得下面的方程组：
[*]
解这个两元一次方程组可得x=60本，y=60本。
