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
答案是E
题目读懂了，能把解题思路告诉我吗，这类题是我的软肋阿

7个里面选一个是C(1,7)

10个里面选2个是C(2,10)

总的选法就是C(1,7)×C(2,10)

达到一个目的分两个步骤，则用乘法公式

第一步骤从7人选1人，7种方法

第二步骤从10人选2人，C 10 2, 45种方法

相乘７＊４５＝E

明白了。谢谢！！！！！