[求助]need your help/大全-2-14

5. 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 time of their accidents. This indicates that, by wearing seat belts, drivers and front-seat passengers can greatly reduce their risk of being severely injured if they are in an auto accident.
The conclusion above is not properly drawn unless which of the following is true?
(A) Of all the drivers and front-seat passengers in the survey, more than 20 percent were wearing seat belts 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-seat passengers in the survey than rear-seat passengers were very 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.

i am not able to understand why A is the correct answer??

如果survey中不代安全带的占x%

不代安全带受重伤可能性＝0.8*survey中受重伤的人／x%*survey的人数

代安全带受重伤的可能性＝0.2*suvey中受重伤的人／(1-x%)*survey的人数

要求：不代安全带受重伤可能性>代安全带受重伤的可能性；所以x%<80%, A

let's say there are 100 people in this survey. 50 of them severely injured and other 50 NOT severely injured. out of 50 those severely injured, 40 of them didn't wear the seatbelt. that's 80%. out of 50 those NOT severely injured, none of them wear the seatbelt. so out of these 100 people, only 10 of them wear the seatbelt at time of accident. that's 10%. but you can still draw the conclusion that "by wearing seat belts, drivers and front-seat passengers can greatly reduce their risk of being SEVERELY injured if they are in an auto accident."

am i missing something??

给个链接你看看

应该差不太多

http://forum.chasedream.com/dispbbs.asp?BoardID=24&ID=44418

thanks, 番茄炒蛋.

it's a good example, but i am not sure if itshould apply to this question. this quesiton really just comes down to crunching the number. and i am still not convinced that A is the correct one...

注题目中的这个表达式of the severely injured drivers and front-seat passengers，在所以受伤的且坐前排的，其中有80%的人，没有带安全带；也就是说，有20%的人带了安全带也受了伤。

而A中，说，在出事的时候，有超过20%的人带了安全带。

也就是说，如果低于20%的人带了安全带。走个极端：假设总共100个人中只有1%人带安全带，那么如果1%带安全带的人100%受了伤，占了全部受伤人数的20%，也就是说有另外99没带安全带的人中有4个人受了伤，显然，带安全带是危险的。

所以，如果原命题成立，则必须要知道，已经带了安全带的比例是多少。