How can one make up for past Ramadan fasts without remembering the number of days missed, and what is its expiation?

Whoever delays making up missed Ramadan days without a valid excuse until the next Ramadan begins, must repent to Allah and strive to determine the number of days that need to be made up, as the Prophet (peace be upon him) said: "Allah's debt is more worthy of being paid back." If she is unable to determine the exact number, she should make up the number that brings certainty or a strong presumption of fulfilling the obligation, in accordance with Allah's saying: "So fear Allah as much as you are able." She must also pay an expiation (kaffarah) by feeding one poor person for each day she delayed without a valid excuse, unless she was unaware of the impermissibility of the delay, in which case there is no expiation upon her.