What is the average number of tweets posted by the users in a city in Argentina?
"Argentina" is the Country; average number of tweets in a city = Divide (Count(TweetID where Country = 'Argentina'), Count (City))
SELECT SUM(CASE WHEN T2.City = 'Buenos Aires' THEN 1.0 ELSE 0 END) / COUNT(T1.TweetID) AS avg FROM twitter AS T1 INNER JOIN location AS T2 ON T2.LocationID = T1.LocationID WHERE T2.Country = 'Argentina'