|re: "let say i have a number in all column from C10 to C30"|
I assume you mean all rows from C10 to C30.
C is a single column, 10 to 30 are the rows.
re: "example in C10 number is 12345.
i want formula in C10 =(12345/12)*5"
I'm not sure what you are trying to do. You can't have a number and a formula in a cell at the same time.
You could put your numbers in Column C and your formulas in Column D:
In D10 you could put =(C10/12)*5 and drag it down to D30.
Is that what you are trying to do?
If you really need to change the numbers in Column C without using another column, there are ways to do that using PasteSpecial so let us know.
BTW...you don't need the parenthesis. Just use =C10/12*5
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