What is the expiation for someone who used to expiate an oath by fasting out of ignorance, despite being able to feed, clothe, or free a slave, and finds it difficult to ascertain the number of past instances?

The expiation for an oath is to feed ten needy persons, or to clothe them, or to free a slave. If one is unable to do any of these, then he must fast for three days. Whoever is able to perform one of the three types [of expiation] is not absolved by fasting, and the expiation remains an obligation upon him. If one does not ascertain the number of expiations [due], then he must take precaution by performing a number that he believes most likely clears his liability.