Since organistation has 30 hosts, Class C network with /24 Prefix should be used since it can support 254 hosts.
Now there are 30 hosts, so the minimum number of bits required for host number is 5, since $2^5 - 2$ is 30.
The rest of bits i.e 3 can be used as subnet number.
So the mask is 11111111.11111111.11111111.11100000
For reference have a look at
http://andthatsjazz.org/wbglinks/pages/reads/misc/ip.html