Among the universities with a score in teaching of over 90 in 2011, what is the percentage of those in the United States of America?
in 2011 refers to year 2011; in teaching refers to  criteria_name = 'Teaching'; score in teaching of over 90 refers to score > 90; in the United States of America refers to country_name = 'United States of America'; percentage refers to DIVIDE(COUNT(country_name = 'United States of America'), COUNT(id))
SELECT CAST(SUM(CASE WHEN T4.country_name = 'United States of America' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) AS per FROM ranking_criteria AS T1 INNER JOIN university_ranking_year AS T2 ON T1.id = T2.ranking_criteria_id INNER JOIN university AS T3 ON T3.id = T2.university_id INNER JOIN country AS T4 ON T4.id = T3.country_id WHERE T1.criteria_name = 'Teaching' AND T2.year = 2011 AND T2.score > 90
database name:db_id university
country table country Cols: id dtype integer, country name dtype text, 
ranking_system table ranking_system Cols: id dtype integer, system name dtype text, 
ranking_criteria table ranking_criteria Cols: id dtype integer, ranking system id dtype integer, criteria name dtype text, 
university table university Cols: id dtype integer, country id dtype integer, university name dtype text, 
university_ranking_year table university_ranking_year Cols: university id dtype integer, ranking criteria id dtype integer, year dtype integer, score dtype integer, 
university_year table university_year Cols: university id dtype integer, year dtype integer, number of students dtype integer, student staff ratio dtype real, pct internation student dtype integer, pct female students dtype integer, 
