IP Subnetting Question

[Robert Cabeca]

I am not understanding the concept of using an IP address followed by /n. Example 10.20.193.20 /28. The way I am looking at it I get a Subnet mask of 255.255.255.240. But I am being told that it is really a mask of 255.255.240.0, however I am not being given an explanation. I thought that 255.255.240.0 has a /20. but it is a /12 instead? Any enlightenment would be appreciated.   peace Rob