一道排列组合题--GWD1-7

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.

7*[(10*9)/2]=315

http://forum.chasedream.com/dispbbs.asp?BoardID=22&ID=174241&replyID=&skin=1

以后做GWD题有疑问时，可先用搜索功能，很多题目前人已提过并有解决方法；如果真的搜索不到，再发贴上来问吧。这样就不用等别人给解答，节约自己的时间了。

方法：如果GWD第3套Q5，则搜索的关键词：3-5

thank you !

怎么做啊