What is the average amount of rent that Christy Vargas paid?
'Christy Vargas' is a full name of a customer; full name refers to first_name, last_name; average amount of rent refers to AVG(amount)
SELECT AVG(T2.amount) FROM customer AS T1 INNER JOIN payment AS T2 ON T1.customer_id = T2.customer_id WHERE T1.first_name = 'CHRISTY' AND T1.Last_name = 'VARGAS'