\[\boxed{\text{779\ (779).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ C_{16}^{4} = \frac{16!}{4! \cdot 12!} =\]
\[= \frac{13 \cdot 14 \cdot 15 \cdot 16}{24} =\]
\[= 1820\ способов.\]
\[\textbf{б)}\ C_{16}^{4} = \frac{16!}{12!} = 43\ 680\ способов.\]