|What Column is the word Rent in? ;-)|
I'll assume A2 with your Monthly/Yearly values in B2 and C2.
You can not put a formula in the same cell as you want to allow user entry. Think about what would happen to the formula when the user manually enters $800. The user would over-write the formula.
In order to accomplish your goal, you will need a macro that monitors both cells and then calculates the values that goes in the other:
1 - Right click the sheet tab for the sheet you want this to happen in and choose View Code.
2 - Paste the following macro into the pane that opens.
3 - Back in the spreadsheet enter a value in B2 or C2. (not A2 or B2)
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Target.Address = "$B$2" Then
Range("$C$2") = Target * 12
ElseIf Target.Address = "$C$2" Then
Range("$B$2") = Target / 12
Application.EnableEvents = True
Keep in mind that macros need to be enabled on any machine that uses this spreadsheet. Macros are something that only the user can enable, they cannot be enabled by a macro. There are ways to "force" a user to enable macros by presenting a "page" in the workbook informing them that macros must be enabled in order for the workbook to be used properly, but we don't have to go into that until you tell me that you need to do that.
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