Among the HTC users, calculate the percentage of female users who are over 80.
HTC refers to phone_brand = 'HTC'; percentage = DIVIDE(SUM(gender = 'F' AND age > 80), COUNT(device_id)); female refers to gender = 'F'; over 80 refers to age > 80
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'