Finite: College Math
posted by Laura .
A student wants to buy 3 CDs, but assume that there are 6 CDs featuring the piano,
4 CDs featuring the trumpet, and
7 CDs featuring the saxophone. In how many ways can the selection be made if
CD's featuring at least 2 different instruments
are selected?

We want selections of the kind
PP?
SS?
TT?
1. 2 piano, one nonpiano > C(6,2)*C(11,1) = 15(11) = 165
2. 2 trumpet, one nontrumpet > C(4,2)*(C(13,1) = 78
3. 3 sax, one nonsax > C(7,2)*C(10,1) = 210
number of selections = 165+78+210 = 453 
453

wrong answer.