I have the following issue :

I have a dropdown with option a, b and c and another with d,e,f. 

Depending on the dropdown choice, a decimal value is assigned to a Decimal Field (say x-factor and y-factor) for each dropdown respectively. 

I then try to multiply these fields in a formula field. 

For some reason I get no value whatever I do. 

If I just type numbers in Decimal Fields x-factor and y-factor I get a value in the formula field !!