Among the employees who work as a Trainee, how many of them have a salary of over &20,000 a year?
Trainee is a position title; salary of over 20,000 refers to salary > '20000'
SELECT COUNT(*) FROM employee AS T1  INNER JOIN position AS T2  ON T1.positionID = T2.positionID WHERE CAST(REPLACE(SUBSTR(T1.salary, 4), ',', '') AS REAL) > 20000 AND T2.positiontitle = 'Trainee'