What is the first valid host on the subnetwork that the node 172.20.225.111 255.255.255.128 belongs to?

Answer:172.20.225.1

subnet mask is 255.255.255.128 so block size will be 

256-128=128

so  Subnetwork will be 172.20.225.0 and first valid host 172.20.225.1