Among the schools' projects whose donation didn't use account credits redemption,how many schools are public magnet schools?
donation didn't use account credits redemption refers to payment_included_acct_credit = 'f'; magnet school refers to school_magnet = 't';
SELECT COUNT(T1.schoolid) FROM projects AS T1 INNER JOIN donations AS T2 ON T1.projectid = T2.projectid WHERE T1.school_magnet = 't' AND T2.payment_included_acct_credit = 'f'