Explanation
Since Johnny has 16 books and wants to select 4 for his vacation, we will have;
[tex]16C4=\frac{16!}{(16-4)!4!}=\frac{16!}{12!4!}=\frac{16\times15\times14\times13\times12!}{12!\times4\times3\times2\times1}=2\times5\times14\times13=1820[/tex]Answer: 1820 combinations
