Column A and B of Sheet1 has the following data entered:
Sheet1 A1=Joe , B1=5
Sheet1 A2=Joe , B2=5
Sheet1 A3=Pat , B3=2
Sheet1 A4=Joe , B4=5
Sheet1 A5 and B5 is blank
Sheet1 A6 and B6 is blank
Sheet1 A7 and B7 is blank

I want to be able to enter the name "Joe" in Sheet5 A2 so that the sum of 15
shows in cell C2 of Sheet5. I need it to work so that if Joe is later
entered in Sheet1 A5 and the number 5 is also entered in Sheet1 B5, Sheet5 C2
changes to the new sum of 20.
How can I do this?