For example I need to be able to put 8 in A1, 7 in A2 and have the total of 15 in A3. I then need to be able to go back and delete the 8 and the 7 and have the total in A3 remain 15. Then I need to be able to put a 7 in A1 an 8 in A2 and have the total in A3 be 30.