请教一道OG上的数学题，我进胡同了

amyyueyue

quantitative 12绿色那本151题：
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eliguble 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 3candidates are there to fill the 3 positions?
A.42 B.70 C.140 D.165 E.315

我的思路：7个candidates选出1个，是7种可能；从十个candidates选出2个给2个positions，我觉得是先选出2个人再分给这两个位置，45*2=90 。看了后面的答案，问题就在于这两个人有没有顺序呢给这两个位置？

求NN解惑一下，十分感谢！

vyc

不是NN，Identical Positions说明不需要排序7*45=315

amyyueyue

谢谢楼上！就是说这两个人对应这两个位置可以随便派，不需要什么顺序，有顺序反而多了可能是吧？