amount calculation using csharp

Question

 no of days in a month textbox calendar(what month i choose that days to be displayed)                                        31

 
except sundays            textbox
(remaining no of days)      26  (this 26 excluded the sundays of what month i choose in the above calendar)
 

basic salary textbox textbox
 
2720/31 * 26 2281
 

similarily i type 2720 divided by what month i choose that day and multiple by excluded the sundays of particular month
and result fetch in to the textbox.
 
2720 is constant amt
.