Heating degree days Calculations

Hi,

Getting close to my exam date! & ... a bit nervous :-]

When do we sum up the heating degree days average together and when we do not ?

On PluralSight videos the questions being asked the same for both scenarios but on one instance the numbers were summing up to the amount of days in a month and in one instance they were just subtracted from 65 degree.

I felt pretty comfortable with this concept after watching the Amber Videos and now I found myself confused again.  

Thanks,