I need an explanation please

1. How many usable hosts per sub-network will you have using a /20 address? ______
2. Given a Class B address, what is the magic number if you need to subnet using a /23 prefix? ______________________
3. In a Class B address, how many sub-networks can you have with a /18 prefix? _______________
4. How many total addresses per network can you have with a subnet mask of 255.255.224.0? (Note that total addresses include network and broadcast addresses.)___________