Question: Among the female users that uses OPPO as their phone brand, what is the percentage of them of the user that uses R815T model to install the app?
SQL: SELECT SUM(IIF(T1.phone_brand = 'OPPO', 1, 0)) / SUM(IIF(T1.device_id = 'R815T', 1, 0)) AS num FROM phone_brand_device_model2 AS T1 INNER JOIN gender_age AS T2 ON T1.device_id = T2.device_id WHERE T2.gender = 'F'