Among the public institutes in the state of Alabama, how many of them have over 30 students who graduated within 100 percent of normal/expected time in 2011?
public refers to control = 'Public'; over 30 students who graduated within 100 percent of normal/expected time refers to grad_100 > 30; in 2011 refers to year = 2011;
SELECT COUNT(T1.chronname) FROM institution_details AS T1 INNER JOIN institution_grads AS T2 ON T2.unitid = T1.unitid WHERE T1.state = 'Alabama' AND T1.control = 'Public' AND T2.year = 2011 AND T2.grad_100 > 30