This example uses A1, B1, and C1.

Put the following event macro in the worksheet code area:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim A As Range, B As Range, C As Range
Set A = Range("A1")
Set B = Range("B1")
Set C = Range("C1")
If Intersect(Target, B) Is Nothing Then Exit Sub
Application.EnableEvents = False
If [C] = "" Then
[C] = [A] - [b]
Else
[b] = [A] - [C] + [b]
[C] = [A] - [b]
End If
Application.EnableEvents = True
End Sub


Because it is worksheet code, it is very easy to install and automatic
to use:

1. right-click the tab name near the bottom of the Excel window
2. select View Code - this brings up a VBE window
3. paste the stuff in and close the VBE window

If you have any concerns, first try it on a trial worksheet.

If you save the workbook, the macro will be saved with it.


To remove the macro:

1. bring up the VBE windows as above
2. clear the code out
3. close the VBE window

To learn more about macros in general, see:

http://www.mvps.org/dmcritchie/excel/getstarted.htm

To learn more about Event Macros (worksheet code), see:

http://www.mvps.org/dmcritchie/excel/event.htm