A recent survey of all auto accident victims in Dole County found that, of the severely injured drivers and front-seat passengers, 80 percent were not wearing seat belts at the tme of their accidents. This indicates that, by wearing seat belts, drivers and front-seat passengers can greatly recduce their risk of being severely injured if they are in an auto accident.

This conclusion above is not properly drawn unless which of the following is true?

a.of all the drivers and front-sea passengers in the survey, more than 20 percent were wearing seat belt at the time of their accidents.

b.considerably more than 20 percent of drivers and front-seat passengers in Dole County always wear seat belts when traveling by car.

c. more drivers and front-sea passengers in the survey than rear-seat passengers were vey severely injured.

d.more than half of the drivers and front-seat passengers in the survey were not wearing seat belts at the time of their accidents.

e.most of the auto accidents reported to police in Dole County do not involve any serious injury.

correct choice: a

前提：在受伤的人群调查中，80％的人没有带安全带

结论：带安全带比较安全。

本题应该是架桥： 即其余20％的人带了安全带。如果只有10％的人带安全带，那说明还有10％没带安全带的都没有受伤，那说明安全带不是不受伤的理由了。

不知道分析的对不对？

80％受重伤的人是没有带安全带的人,

即其余20％受重伤的人是带了安全带的人,那就是说有20％以上的人带了安全带---戴安全带的没有都受重伤.

(要是20％的人带了安全带,刚好这20％的人受重伤,那成什么了?带了安全带的人100%都受伤了!要是有少于20％的人带了安全带,怎么会有20％受重伤的人是带了安全带的???)

这简直是数学题!

具体数据然后去极端，设受调查的人总共是1000，严重受伤的人是100

>20%的受调查人系安全带取极端有90%的人系安全带，则900个人里有20个人严重受伤，剩下100人里80个人就严重受伤，所以避免严重受伤的方法是系安全带。

系安全带受重伤的可能性会小很多，比较安全．没有绝对安全.

和安全带性能怎样没关系  只要系安全带的人数总和（受伤+未受伤）大于系安全带受伤的就是答案

我想通了。以下是我的想法，不知道对不对。

化为算数题：

假定参加调查的1000人，严重受伤的100人

按A假设：1000人中有超过20%的人系了安全带，我们取20%先算，即200人系了安全带，800人未系安全带。

100严重受伤的人中20人系了安全带，80人未系安全带。

算系安全带严重受伤的人的比例为20/200=1/100，  未系安全带严重受伤的人比例为80/800=1/100

以上两个比例在1000人中只有20%的人系安全带的情况下相等。所以如果要得出系安全带更安全的结论 以上比例前者要小于后者，即系安全带的人要超过200人才可。