Hi,

Edit Beløb field settings, choose Formula instead of Number, put your formula ( #Beløb F + #Beløb S ) in formula editor window. If field name contain space or some non-letter chars, it needs to be wrapped like this:
{Beløb F} + {Beløb S} . Just start and then click on hints, you will see it's very easy.

Thanks a lot. 

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