Why do my formulas work in some cells but not others?

[Lowly Worm]

Excel 2003 (and earlier versions)
I frequently run into the problem where I type a formula in one cell and it
doesn't work (all I see is the text of the formula in the cell), yet I can
type the exact same formula in a different cell on the same spreadsheet and
get it to calculate.

For example - I'm trying to use this formula (where XA:XZ represents any
random range):

=SUMIF(XA:XZ,">750000")

And it won't work in all the cells. I've tried clearing the contents and
also deleting the cell, but nothing works. Any suggestions?

 

[JMB]

the cell is probably already formatted as text. change the format to
general, then edit the cell (F2) and hit enter, or retype the formula and hit
enter.

 

[Sloth]

Sounds like the format for that cell is set to text. Change it to general or
number.

 

[Lowly Worm]

Thanks for your help everyone! This did the trick!