请各位大侠指点一二

yin0883

one-fifth of the light switches produced by a certain factory are defective.Four-fifths of the defective switches

are reject and 1/20 of the nondefective switches are rejected by mistake. if all the switch 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%

为什么答案是B啊？

amumu

(1/5*1/5) / (1/5*1/5 + 19/20*4/5) = 1/20 = 5%

[此贴子已经被作者于2009/9/25 18:04:42编辑过]

yin0883

明白了，谢谢了！