Write SQL query to solve given problem: What is the average number of tweets posted by the users in a city in Argentina?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'