She can fast three days of every month and she will get the same reward of 6 of shawwal

I know that we have to make up our missed days of ramadan and then fast the 6 days of shawal. My sister doesn’t have time to fast first her missed days and then the 6. That is why she asked if she can fast the 6 days first before shawal ends and then her missed days. Is it permissible?

Fasting | July 13, 2017 4:06 am
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I have 8 missed fasts due to menstruation. Can i fast them now and intend to fast the 6 days of shawwal too? Meaning i am paying back while fasting sunnah too?
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If someone wishes to fast the 6 days of shawwal but cannot fit them in, can they fast after shawwal those fasts or do they lose them?