Among the coaches whose team has over 30 wins in a year, how many of them are born in the USA?
over 30 wins refers to w>30; born in the USA refers to birthCountry = 'USA'
SELECT COUNT(T2.coachID) FROM Master AS T1 INNER JOIN Coaches AS T2 ON T1.coachID = T2.coachID WHERE T2.W > 30 AND T1.birthCountry = 'USA'