how to calculate the percentage of amount using csharp code

Question

basic 2720 2193 for both amt i use text box.
2193 how i get it means (2720/31)*25
2266 how i get it means (2720/30)*25
 
how to do the above one.suppose if i type 31 means 2193 to be arrived
and if i type 30 means 2266 to be arrived in the textbox.


rgds,
Narasiman P