Declaring variables in Excel Cells
Solution 1:
You can name cells. This is done by clicking the Name Box (that thing next to the formula bar which says "A1" for example) and typing a name, such as, "myvar". Now you can use that name instead of the cell reference:
= myvar*25
Solution 2:
I also just found out how to do this with the Excel Name Manager (Formulas > Defined Names Section > Name Manager).
You can define a variable that doesn't have to "live" within a cell and then you can use it in formulas.
Solution 3:
The lingo in excel is different, you don't "declare variables", you "name" cells or arrays.
A good overview of how you do that is below: http://office.microsoft.com/en-001/excel-help/define-and-use-names-in-formulas-HA010342417.aspx
Solution 4:
You can use (hidden) cells as variables. E.g., you could hide Column C, set C1 to
=20
and use it as
=c1*20
Alternatively you can write VBA Macros which set and read a global variable.
Edit: AKX renders my Answer partially incorrect. I had no idea you could name cells in Excel.