\[a_{1} = 16;\ \ \ a_{n + 1} = \frac{a_{n}}{2}\]
\[a_{2} = \frac{16}{2} = 8\ ;\ \]
\[a_{3} = \frac{8}{2} = 4;\ \ \]
\[a_{4} = \frac{4}{2} = 2.\]