need to reduce the value for runtime in gridview in asp.net

Question

need to reduce the value for runtime in gridview in asp.net

i have bind the value as billamount in grid and i have the textbox and i enter the value in runtime i need the output in the grid.

Name	Date	billamount
Celv	14/4/15	15000
celv	25/4/15	25000

Now In runtime am giving amount as 20000 .

Name	Date	Billamount
Celv	14/4/15	15000	20000	5000
Celv	25/4/15	25000	5000	20000

I need to reduce the amount 5000 for the next date in asp.net