If a lady doesn’t have a calculation of how many fast was qadha due to menstruation but she use to keep a full day roza in the recommended days of mahe Rajab and Shaban..
Then what is the ruling. How should she go about it.

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She must have the calculations of the qadha roza. According to her dates try to calculate.
First of all if one has to pay qadha of fast that person is not allowed to fast recommended one. In the past if she did fast mustahab, that will not transfer as a wajib. She will be rewarded for Mustahab fast and wajib she has to pay