|My guess is that the cell have been formatted as text and will not let go of the format.|
1 - Delete the formula and temporarily put a number in the cell.
2 - Put a 1 in any other empty cell
3 - Select the cell with the 1 and do a Ctrl-C to copy it
4 - Select you list of "currency cells", including the fell where you want your formula.
5 - Right-click anywhere in the selected cells and choose PasteSpecial
6 - In the Operation section, chose Multiply, click OK.
Hopefully this will force a mathematical operation on the cells and convert them to numbers.
7 - Try your formula again.
If that doesn't work, it's possible that your options are set to Show Formula, but I doubt that that is the case.
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