题目不会~求救`

littleray

555```````,这个题目又不会,谁能给我讲讲啊?

One-fifth of the light switches produced by a certain factory are defective. Four-fifth of the defective switches are rejectedand 1/20 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?

答案是4%,是不是1/5乘以1/5得出来的啊?为什么呢?怎么我自己计算的不是这个呢?

好心人和我讲讲好吧?

多谢了~

mindfree

Defect -- D   Nondefect -- N   Total -- T   Rejected -- R   Sold -- S

D+N=T; D = 1/5 T; N = 4/5 T; ==> N = 4D

4/5 D + 1/20 N = R ==> S = T - R = D + N - 4/5 D - 1/20N = 1/5 D + 19/20N

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

Why not 4%? I do not know.  

lawyer_1

这好像不是逻辑题

1。如果问题是：生产的SWITCHES中，多大比例的有缺陷SWITCHES被卖出去。答案则为4%

2。如果问题是：卖出去的SWITCHES中，多大比例是有缺陷的，则答案为5%

littleray

是不是说这个问题的问题指代不是很明确,有两个可能性啊?
谢谢大家对我的帮助,以后还可以在这里贴数学问题吗?