Create a named range on the cell that you are wanting to add 10 to, I called that range rngCellToAdd10To ...
Then add the below code (using the VBA editor) to the worksheet object that the cell you want to monitor changes for ...
Private Sub Worksheet_Change(ByVal Target As Range)
On Error Resume Next
If Intersect(Target, Range("rngCellToAdd10To")) Then
Application.EnableEvents = False
Range("rngCellToAdd10To").Value = Range("rngCellToAdd10To").Value + 10
Application.EnableEvents = True
... then add a number to the cell and watch it go.
If you're not sure how to add code into the VBA editor, I suggest Googling it.