天山4-Q35:

jonathanher

For what percent of those tested for a certain infection was the test accurate; that is,

positive for those who had the infection and negative for those who did not have the

infection?

(1) Of those who tested positive for the infection, 1/8 did not have the infection.

(2) Of those tested for the infection, 90 percent tested negative.

答案是E，我的答案是C。

jonathanher

顶起来，大牛帮忙解答下啊！先谢了！

littlegirl

分四种情况：正 感染  ； 正  没感染  ； 负 感染   ； 负  没感染

                    正确                                                                   正确

              

 条件一：“正感染” 和 “正没感染”的1/8是“正没感染”

          即：       7            ：       1

 条件二：“正感染”和“负感染”的90%是“负感染”

          即：    1                       ：                    9 

求正感染比例，少条件，选E