"Good afternoon, how are you?"

Translation: Buon pomeriggio, come stai?

[pr47]

Duolingo is not teaching dialect, or local custom. Buon pomeriggio is the literal translation. Whether or not it is commonly used in a certain region or province is of no signifigance in this type of setting.

[Cinciarella]

Also "buona sera" or "buonasera" should be quoted among the correct answers; in the (late) afternoon it's correct to use this expression.

[lblacorte]

The difference between stai and sei in this instance, anyone?

[RalphSherwin]

buona sera is perfectly correct

[fJLo]

Would it be buon sera in the subject and buona sera in the object?