Given it is a class B network, so the subnet mask will be something like 255. 255. _ . _
The question says that we need 220 systems in each subnet.
Using 8 bits, we can have $2^8 - 2 = 254$ systems(hosts). While with 7 bits we can have only $2^7 - 2 = 126$ systems.
So, we need to have atleast 8 bits for host address in each subnet. This means that the last octet of IP address should be 0. Our answer now will be something like: 255. 255. _ . 0
Given that we need 200 subnets. Using 7 bits we can only have $2^7 = 128$ subnets, so we need atleast 8 bits for subnet $(2^8 = 256 > 200)$.
This means that atleast 8 bits are required for subnet addressing.
Therefore subnet address should be 255. 255. 255. 0
(Last 8bits are used for host addresses).
