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If your health allow you to fast without any health complexities and weakness, you can fast as Qadha as much as you can do easily.
If you had missed the fasting because you were not able to do fasting then only Qadha or Fidya is wajib not Kaffara.
But if you had missed them intentionally or carelessly while you were able to do fasting in your healthier lifetime then make sure you pay for both Fidya and Kaffara for each missed fasting.
Once you paid Fidya and Kaffara of each day of missed fasting then you do not need to do Qadha anymore.
Fidya is an amount equal to three quarters of kilogram (3/4) of wheat/rice for each single day missed fasting. And Kaffara is to feed 60 poor people for each missed Fasting. This amount should be given to the deserving poor momin/momineen. Or you can send to Imam al-Khoie Islamic Center.