ANSWER:

She must estimate the numbers missed and add a couple more to be on the safe side and make them all up. 

My sister missed many Ramadan fasting for past few year due to her period,and she didn’t make up for that later, thinking that in this time if we miss salah we dont have to make up.Now she knows that she have to make up the missed fasting, But how she will make up for the previous years fasting?

Fasting |