Database: student_club
Question: Among the members with t-shirt size of medium, what is the percentage of the amount 50 received by the Student_Club?
SQL: SELECT CAST(SUM(CASE WHEN T2.amount = 50 THEN 1.0 ELSE 0 END) AS REAL) * 100 / COUNT(T2.income_id) FROM member AS T1 INNER JOIN income AS T2 ON T1.member_id = T2.link_to_member WHERE T1.position = 'Member' AND T1.t_shirt_size = 'Medium'