问题
单项选择题
A decorator bought a bolt of defective cloth that he judged to be 
usable, in which ease the cost would be $0.80 per usable yard. If it was later found that only 
of the bolt could be used, what was the actual cost per usable yard ？（）
A.$0.60
B.$0.90
C.$1.00
D.$1.20
E.$1.70
答案
参考答案：B
解析：
设这匹布共有a码，每码可以用的布的实际成本为x美元，则根据题意可得
。