What is the prefix length for the following subnet masks? Prefix Length How many hosts are available?

$255\mathrm{.}255\mathrm{.}255\mathrm{.}192-$ There are $60$s in the host portion so use the formula: $2^6=64($There are $64$ hosts available$)$

$255\mathrm{.}255\mathrm{.}255\mathrm{.}240-2^?=$

$255\mathrm{.}255\mathrm{.}0\mathrm{.}0-2^?=$