What is the percentage of Professor Ogdon Zywicki's research assistants are taught postgraduate students?
research assistant refers to the student who serves for research where the abbreviation is RA; taught postgraduate student refers to type = 'TPG'; DIVIDE(COUNT(student_id where type = 'TPG' and first_name = 'Ogdon', last_name = 'Zywicki'), COUNT(first_name = 'Ogdon', last_name = 'Zywicki')) as percentage;
SELECT CAST(SUM(CASE WHEN T3.type = 'TPG' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.student_id) FROM RA AS T1 INNER JOIN prof AS T2 ON T1.prof_id = T2.prof_id INNER JOIN student AS T3 ON T1.student_id = T3.student_id WHERE T2.first_name = 'Ogdon' AND T2.last_name = 'Zywicki'