IF function in Data Validation- XL03

[Guest]

I want to make a cell only validate an entry if another cell equals a value.
I'm using DATA>VALIDATION and selecting CUSTOM. This is my formula:

=IF(NOT(C5="R"),"X","")

I want this cell to recognize if Cell C5 contains ONLY the letter "R" it is
to accept ONLY the letter "X" as an entry. If cell C5 doesn't contain ONLY
the letter "R" it is to accept ANY entry.

 

[Guest]

You need a formula that returns TRUE when either C5 is not R or the given
cell is set to X. If this is to apply to A1, then the custom validation
formula would be
=or(C5<>"R",A1="X")