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?
42
70
140
165
315
发现这种选人的都是用乘法,比如A组8人里选2个 B组3人里选2个,就是 C(8,2)* C*(3,2)
N=7C1*2C10=7*45=315
we use 2C10 rather than 2P10 because "2 of 10 candidates eligible to fill 2 identical positions in the computer science department