how to sum a particular column in a datagrid

can someone pls help me.. i want to know how to sum a particular column in a datagrid let say for example i have a datagrid that contains all the column "ID","Food","Amount" how can i sum the "Amount" column by clicking a combox that i also use for search of a particular food.. ie. ID FOOD AMOUNT 1 aa 50.00 2 bb 55.00 3 bb 55.00 if i select "bb" from a combox it should total the amount of the food which is 110.00 hope you can help me tnx...