Computer Networking – Determining the range of addresses

I could use some help with this problem:

An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for two organizations each with 500 addresses, two organizations each with 250addresses, and 3 organizations each with 50 addresses. Give a table that shows the range of addresses allocated to each organization. For each of the blocks (the 7organizations and the unused block(s)), indicate the size of the block, the first address in the block, and the last address in the block using CIDR notation.

I will rate lifesaver for a well-explained answer! Thanks for your effort!