Total number of CDs = 12
Number of CDs that can be selected at once = 2
Selecting 2 CDs from 12 means combination of 12 objects taken 2 at a time. This can be calculated by 12C2.
[tex]12C2 = \frac{12!}{2!*(12-2)!} \\ \\
12C2= \frac{12!}{2!*10!} \\ \\
12C2= \frac{12*11}{2} \\ \\
12C2=66[/tex]
This means, we have 66 possible combinations to choose from
