Retrieve field value from GridView to compute total

[dfs3000my]

Hi, attached in the attachment below has a screenshot of what I am trying to accomplish... I have hurdle marks for individual students and i am trying to compute the total of those marks.

How can I access those values, compute the total, and store them back in the "Total Marks" column and as well as computing the "Grades" depending on what mark they obtain.

Example: 80 - 100 : HD
70 - 78 : DI
0 - 69 : Fail

[carobee]

Are the columns Total and grades created through template?

[dfs3000my]

Are the columns Total and grades created through template?

They are created as bound fields in the code-behind code which has no data field. that's why they are currently blank.