Form Calculations

I have Acrobat XI Standard installed and use it rarely.  I am having problems calculating a value.  The formula used is AdultCost = Adults * CostPerAdult.  All three are text fields defined as numbers.  When I right click the AdultCost field and select properties, on the Text Field Properties Calculate tab, I am unable to select individual text fields from the list of text fields which is displayed after clicking the button to the left of “Value is the”.  I can select or deselect all fields buy can not select or deselect an individual field.

Can anyone recommend how to resolve this calculation issue?