What is the percentage of American players among all the players who have gotten in the Hall of Fame?
percentage of American players = divide(count(hofID where birthCountry = 'USA'), count(hofID))*100%
SELECT CAST(COUNT(CASE WHEN T1.birthCountry = 'USA' THEN T1.playerID ELSE NULL END) AS REAL) * 100 / COUNT(T1.playerID) FROM Master AS T1 INNER JOIN HOF AS T2 ON T1.hofID = T2.hofID