subnets for 64 departments
means
you need 6 bits for subnets
here by 6 bits we mean with 6 unique bits we can have 64 sub networks
classB
16 bits NID rest 16 bits host ID
first 6 bits of the third octet will be used
mask
11111111.11111111.11111100.00000000
255.255.252.0