Habt ihr gestern gearbeitet?
Did you work yesterday? - Remember, I am slow when it comes to grammar. So is the above sentence formal, informal or none of the above?
I believe it is informal plural.
Hast du gestern gearbeitet? = singular, informal "you"
Habt ihr gestern gearbeitet? = plural, informal "you", such as asking a group of people whom you would use "du" with individually
Haben Sie gestern gearbeitet? = singular or plural formal
I think Duolingo as a framework doesn't adapt too well to the specific pecularities of the various languages in order to support a lot of languages.
But as a general tip: if the pronouns are capitalised (and do not stand at the begining of the sentence), then it is always the formal form.