请教 tt-6-7,tt-6-14

其中绿色为正确答案

Q7:

A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department.  If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?

A.     42

B.     70

C.     140

D.     165

E.      315

Q14:

A total of 30 percent of the geese included in a certain migration study were male.  If some of the geese migrated during the study and 20 percent of the migrating geese were male, what was the ratio of the migration rate for the male geese to the migration rate for the female geese?

[Migration rate for geese of a certain sex = (number of geese of that sex migrating) / (total number of geese of that sex)]

A.     1/4

B.     7/12

C.     2/3

D.     7/8

E.      8/7

上班时间偷个懒：

1。经典概率 C1/7*C2/10=7*45=315

2. 设migrated geese为X，

male=0.2X/30, and female=0.8X/70

ration=7/12

总g  x

male: 30% x

female: 70%x

总migrant y

mm : 20% y

mf : 80% y

然后就出来了

我就纳闷，

1。经典概率 C1/7*C2/10=7*45=315

怎么区别 C1/7*c1/10*c1/9?????

总是弄混~~~

这两题的错误原因，靠，又是中文翻译错误，

第一题，我翻译成从1到7号里选1个a,从2到10号里选2个b

然后得出 c(1,1)*c(2,9)+c(1,6)*c(2,8) 还有另外一个公式，两个答案一样，记不清了，还特美。。。结果一个答案都没有

第二题，翻译ratio的时候没看该ratio的定义，怪不得以前做这个题很轻松，后来做这个题得不出答案。。。

看来只睡3个小时后做题真的是头脑混乱，不过齐丹被罚下的时候感觉确实怪怪的。。。

也就是分不清C2/10跟C1/10×C1/9啦，这个简单，前者是表示无序的选择，也就是说只要选两个人出来就可以了，不用管顺序，ab和ba算一种；后者是表示有序选择，也就是选出两个人还得按顺序，ab和ba算两种。

再简单一点说，C1/10×C1/9就等于P2/10嘛，你看怎么区别？