A small organisation has a class c addresses for 7 network, each with 24 hosts, what is an appropriate subnet mask?

A Class C address requires 21 bits for its network ID, leaving 8 bits for the host ID and subnet ID to share. One possible scheme would assign 4bits to the host and 4 to the subnet ID, as shown below
Network-id : 0- 23
Subnet-id    : 24 -27
Host-id       :  28 31



Subnet mask: 255.255.255.224