How to use events?

Add the amount according to PO no & line item

sppose PO no Line Item amount

46 30 20

46 10 20

46 10 30

47 10 10

47 20 10

No i want output

PO no Line Item amount

46 30 20

46 10 50

47 10 10

47 20 10

what events i can use for it like At new ,on change of or at end of