A magnetic ciruit , composed of four quarter-rings of different core materials, is joined at the ends to form a doughnut the cross-sectional area of the core is 0.05, and the reluctances of the four quarters  are 1500, 1350 , 1250 and 900  respectively, A coil of 900 turns and 8 resistance is wound around the douhgnut and connected to a 72 battery . Determine

여기서 자속 core flux를 구하라고 하는데 아예 감이 안잡히네요 .. 전제 릴럭턴스가 5000 인거는 알겠는데 F=자속*릴럭턴스로 구하는거 같은데

F를 못찾겠어요 힌트라도 주시면 감사하겠습니다.