Question: What percentage of customers who paid with a Discover Credit Card gave a 3-star rating?
SQL: SELECT CAST(COUNT(CASE WHEN T1.StarRating = 3 THEN 1 ELSE NULL END) AS REAL) * 100 / COUNT(T1.CustomerID) FROM rootbeerreview AS T1 INNER JOIN `transaction` AS T2 ON T1.CustomerID = T2.CustomerID WHERE T2.CreditCardType = 'Discover'