VB/ Formula advice needed

Hello, i hope someone can help

Ive got a spreadsheet A1 + B1 = C1 (A1 sources data from other sheets). I want to be able to allow the user to put a "score" in B1 so it changes the total of C1 but then i want the cell to clear but it not affect C1 total ready for the next score to be put in. I would ideally like to be able to do it on a drop down box or something similar but dont know where to start. I hope someone can understand this. Thanks for any help.