Question: Among the HTC users, calculate the percentage of female users who are over 80.
SQL: SELECT SUM(IIF(T1.gender = 'F' AND T1.age > 80, 1, 0)) / COUNT(T1.device_id) AS per FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'HTC'