Among the projects whose donators are teachers, what is the percentage of projects that affected more than 30 students?
donors are teachers refers to is_teacher_acct = 't'; affect more than 30 students refers to students_reached>30; Percentage = Divide(Count(students_reached>30), Count(students_reached))*100
SELECT CAST(SUM(CASE WHEN T1.students_reached > 30 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.projectid) FROM projects AS T1 INNER JOIN donations AS T2 ON T1.projectid = T2.projectid WHERE T2.is_teacher_acct = 't'