What is the method for making up for 20 years of missed fasting, and is there an alternative to fasting or something alongside it, and how can the days and the amounts due be calculated, and what is the ruling on interrupting expiatory fasting due to menstruation, Eid, or forgetfulness?

We advise you to repent sincerely and hasten to it, for Allah accepts repentance. You must make up for all the Ramadan fasts you missed from the onset of puberty. The make-up fasts do not need to be continuous. If you are unable to make them up, then you must feed one poor person for each day (approximately 750 grams of the staple food of the country). As for expiation (Kaffarah) due to breaking the fast in Ramadan, it is only obligatory for intentional sexual intercourse during the daytime. It is not obligatory for intentionally eating or drinking. If one eats or drinks out of forgetfulness, their fast is valid. If intentional sexual intercourse occurred and you were coerced, there is no expiation upon you. If you were willing, the majority of scholars hold that expiation is obligatory, which is freeing a believing slave. If you are unable to do so, then fasting for two consecutive months. If you are unable to do that, then feeding sixty poor people (750 grams of the staple food of the country for each poor person). The days of menstruation are included in the period that must be made up. If you fast for two consecutive months for expiation, then make up the period during which menstruation occurs. You must make up for the days of Eid and the days of Tashreeq; breaking the fast on these days does not interrupt continuity. It is disliked to fast on the Day of Doubt unless it is a make-up fast, a vow, or a voluntary fast. If you are unable to ascertain the exact number of days, then continue making them up until you are certain.