sql - how to do calculations with qelement results in xml? -

i have xml code returns result sql query:

<qelement fname = "machine_dt" dbname="db">   declare @dt int         set @dt =(select top 1 [value] [db].[dbo].[configdbe] [line] = '1' , [name] = 'hello')   select isnull(@dt,0) </qelement>  

how can use result perform calculations it?