求助一道百分比的逻辑题

yeah529

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.


19. The argument above depends on which of the following 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.
(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.
(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.
(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.
(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors.


A是如何得出来的呢。。还有题干里的意思是虽然新的检查系统能排除5%的捐献者，但是他们仍然会漏掉三分之二的带有XX病毒的捐献者。是这个意思么？？=  =不理解这两个百分比有什么用啊

yeah529

自顶一下！

睿宇

假设300人捐血接受肝炎疫苗检测，按照题意，估计能够被检测出的为5%，即15人。根据“they will still miss two-thirds of donors carrying NANB hepatitis” 可得实际患病未被检测出来的为2/3,即检测出来的15人所占比例为1/3,那么实际患病人数为15/(1/3)=45人。THEREFORE,还有30/300=10%的人未被检测出来但患有肝炎疾病的人依然在献血。
假设的条件说：检测试验测得的捐血人患肝炎未感染其他疾病。
如果没有这个假设，disqualify up to 5 percent of all prospective blood donors就可包含因为其他疾病而不合格的患者。既而就不能进行接下来的运算了。

yeah529

假设300人捐血接受肝炎疫苗检测，按照题意，估计能够被检测出的为5%，即15人。根据“they will still miss two-thirds of donors carrying NANB hepatitis” 可得实际患病未被检测出来的为2/3,即检测出来的15人所占比例为1/3,那么实际患病人数为15/(1/3)=45人。THEREFORE,还有30/300=10%的人未被检测出来但患有肝炎疾病的人依然在献血。
假设的条件说：检测试验测得的捐血人患肝炎未感染其他疾病。
如果没有这个假设，disqualify up to 5 percent of all prospective blood donors就可包含因为其他疾病而不合格的患者。既而就不能进行接下来的运算了。

-- by 会员 睿宇 (2011/10/24 21:33:10)

如此复杂=  =我都觉得我木时间算那么清楚唉。。是不是遇到百分比的题目就尽量要找有百分比的选项呢？？？

fox0923

为什么C不可以？是因为the number of donors吗？而原文说的是percentage。