What is the average number of reviews written for active businesses that operate not more than 30 hours a week?
avg(user_id) where active = 'true' and SUM(SUBTRACT(closing_time, opening_time)) < 30;
SELECT AVG(T3.user_id) FROM Business AS T1 INNER JOIN Business_Hours AS T2 ON T1.business_id = T2.business_id INNER JOIN Reviews AS T3 ON T1.business_id = T3.business_id WHERE T1.active = 'true' GROUP BY T2.closing_time - T2.opening_time HAVING SUM(T2.closing_time - T2.opening_time) < 30