问题
单项选择题
One-fifth of the light switches produced by a certain factory are defective. Four-fifths of the defective switches are rejected and 
of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective ？（）
A.4%
B.5%
C.6.25%
D.11%
E.16%
答案
参考答案：B
解析：
根据题意可得出售的有缺陷的开关占总量的比率
。
出售的无缺陷开关占总量的比率
。
则出售的开关中有缺陷的开关所占的比例为
。