title: how to get percentage of two numbers and calculate from a percentage
body: <p>I am developing a application that need to calculate percentage from two numbers. dotnet have some method for do this? i like to write this calculate by myself. but i don't good in mathematics unfortunately.
  really, how to get percentage in mathematics? for example i have 10000 request in my application, and 400 request of them are for special part of my application. how to calculate what percent of request are request for a part of my application?
  and how to calculate reverse, for example, i know 5 percent of users like my special post, how can calculate number of users that like that post?
  Thank you and sorry for my bad English.</p>