Find the common ratio.

$\frac{4}{8}=\frac{1}{2}$

$\frac{2}{4}=\frac{1}{2}$

Use the equation for an infinite geometric series:

$\displaystyle\sum_{n=1}^{\infty}{ar^{n-1}}=\frac{a}{1-r}$

$16$

See part (a).

Not possible. $r>1$, and the terms keep increasing.