Sorry if this question has been answered

* MS – Office 2007

I have 3 cells in a worksheet
Column a: 500
Column b: 475
Column c: 25

Column (a) never changes.
The values in columns (b and c) will change weekly. I would like to update (column c) and have (column b) adjust accordingly to not exceed column (a).

Would I do this through code or is there a simpler solution.