State the number of users who are under 50 and above 20 use device model of Galaxy Premier.
under 50 and above 20 refers to age BTWEEEN 20 AND 50;
SELECT COUNT(T1.device_id) FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.age BETWEEN 20 AND 50 AND T2.device_model = 'Galaxy Premier'