Write SQL query to solve given problem: Among students with low salary, how many of them have a gpa of 3.5?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT COUNT(T1.student_id) FROM RA AS T1 INNER JOIN student AS T2 ON T1.student_id = T2.student_id WHERE T2.gpa = 3.5 AND T1.salary = 'low'