求助 假设 百分比类型的题目！！

hanlei84921

1. Twenty percent of all energy consumed in the United States is consumed by home appliances. If appliances that ate twice as energy-efficient as those currently available are produced, this figure will eventually be reduced to about ten percent

The argument above requires which of the following assumption?

(A)Home appliance usage would not increase along with the energy efficiency of the appliances

(B)It wouldn't be expensive to produce home appliances that are energy-efficient

(C)Home-appliance manugacturers now have the technology to produce appliances that  are twice as energy-energy those currently available.

(D)The cost of energy to the consumer world rise with increase in the energy efficiency of home appliances.

(E)The percentage of energy consumed by home appliances will increase if existing appliances are not replaced by more energy-efficient models.

答案是A。这题涉及到百分比换算，所以不是很懂。怎样入手呢？

2.Inspection system X and inspection system Y, though based on different principles, each detect all product flaw but they each also erroneously reject three percent of flawless products. Since false rejections are very costly, monkey will be saved by installing both systems, instead of either one or the other, and rejecting only products found flawed by both.

(A) The three percent of flawless products that system X rejects are not all the same products, piece for picee, which system Y erroneously rejects.

(B)It is less costly to accept a flawed produc than to reject a flawless one.

(C) In their price range, system as X and Y are the least error-prone inspection systems on the market.

(D)Whichever system performs the second inspection needs to inspect only products not rejected by the first system

(E)Any way of detecting flaw,other than by using either system X or system Y, requires complete disassembly of the products.

答案是A，又该如何计算？

3. 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 passenger in the survey, more than 20 percent were wearing seat belts at the time of their accidents.

这题答案就是A，怎么计算呢？

4.Blood banks will shortly start to screen all donors for NANB hepatitis. Although the new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors, they will still miss two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood.

Assumptions?

(A)Donors carrying NANB hepatitis do not, in a large percentage of cases , carry other infections for which reliable screening tests are routinely performed.

答案也是A,

这四题都是假设里的百分比的题目，一碰到就头大，求助大家，这四题怎样做？以后怎样做这种百分比的题目？

pfullsomer

（3）这题我也错过。我把我的思路写出来，看对不对。
        从题目可知，重伤的人当中，80%没戴安全带，即20%有戴但仍然重伤。结论是戴安全带能大大降低受重伤的风险。
        注意A的描述，是all the drivers and front-seat passengers in the survey，我认为在这里范围发生了变化（假设所有受伤的drivers和front-seat passengers按伤势轻重分为severly injured和lightly injured两组）在重伤组中戴安全带受重伤的概率为20%，如果整个调查对象（两个组）有超过20%的人戴安全带，则可推出在轻伤组中戴安全带受轻伤的概率>20%（假设有30%的人戴安全带，(S+L)*30%=S*30%+L*30%，那么令(S+L)*30%=S*20%+L*x%，则可肯定x>30%>20%，再假设S和L人数一样，则x=40%）由于戴安全带的人当中会受重伤的概率是20%，受轻伤的概率是40%，那么戴安全带更可能受轻伤，即降低了受重伤的概率。

另，对D选项取not，即变为和A相同的模式，套用同样的思路，为加强结论，因此不能作为assumption

其他选项均为无关选项。

pfullsomer

(1) 现时的电器消耗美国20%的能源。如果电器效率是现时的两倍，那么用电量会减少为全美的10%。用电量的变化只能受效率的影响，assumption要排除其他原因，例如，电器的使用时间（如果效率提高，但使用时间变长，则削弱结论）

（2）取非：如果X和Y都排除的产品一样，那么装两个跟装一个没区别，削弱结论。成立。