How to aggregate set of marks from database in asp uisng c#

hi friends,

 please help me in aggregating set of marks from database. when i select the a particular department using dropdown listbox then the percentage of all the students has to be appear in the gridview who belongs to that branch in asp using c#.

 for example.

  it i select cse department then rollno,name,percentage has to be appear in the gridview. similarly other branches also .

my database is shown below.
----------------------------

database table
rollno	name	c	dbms	java	se
1	chiru	80	89	78	88
2	venu	89	88	78	67
3	naveen	77	67	75	65

now  it should display the above data percentage like shown below

rollno	name	percentage
1	chiru	83.75
2	venu	80.5
3	naveen	71

thanks in advance