[求助]GWD 涛涛 15-35

Q35:E???

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.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

这题的答案是E，为什么不是C呢？

Answer:

应该是选E的.

分析1)在测出有病的人中,1/8(也就是12.5%)的是错误的,也就是87.5%是正确的

     (2)在所有被检测的人中,检测结果为:90%人没病

由(2)得出:被检测的人中,100%-90%=10%的人是有病的

(1)和(2)单独都推不出来,(1)+(2)只能知道:

被检测有病的人中,正确的占总人数的10%*87.5%=8.75%

但是原题问:所有被检测人的正确率为多少

我们并不知道被检测没病的人中,正确率为多少

所以这道题选E

终于有人理我了！

多谢MM，俺明白乐。。。