Jack, a network technician, needs to subnet a class C network into as many subnets as possible, with at least 22 usable hosts per subnet. What subnet mask should he use?
255.255.255.224 is the correct subnet, because it gives 30 usable hosts per subnet, and will give you the most possible subnets.
Wikipedia
A subnetwork or subnet is a logical subdivision of an IP network The practice of dividing a network into two or more networks is called subnetting Computers that belong to a subnet are addressed with an identical most-significant bit-group in their IP addresses This results in the logical division of an IP address into two fields: the network number or routing prefix and the rest field or host identifier The rest field is an identifier for a specific host or network interface The routing prefix may be expressed in Classless Inter-Domain Routing (CIDR) notation written as the first address of a network, followed by a slash character (/), and ending with the bit-length of the prefix