Answer:255.255.255.224
Explanation:
/20 has a subnet mask of 255.255.240.0
We have 20 on bits for the network and 16 off bits for the host. Since the designer wants 20 to 30 hosts in each subnet we will be having 5 off bits for the host. Therefore we will be having a /27..
Which will be 255.255.255.224