how to export data grid view into excel and validate c#

Question

Hello friends
I m facing a very Big problem right now actually I have transaction form which is bind by data grid view.and all the columns is validated in data grid view now when I m export it all the validation is not coming in excel for example how to define range in cell or columns in excel ...
If user enter 10 marks from 10 then grade should be come A in next cell so I don't have any idea how to make it in excel file.... Hope u r understand what I want ...

Thanks in advance