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

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

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.