Is one excused for ignorance when breaking an oath, and is expiation due if the one who broke the oath was ignorant of his violation? And is feeding twenty sandwiches sufficient as expiation for an oath?

The questioner must offer expiation for his oath, and ignorance of the ruling does not exempt him from it, because it was incumbent upon him to ask people of knowledge before breaking his oath. What the questioner did by offering expiation is incorrect, neither in terms of type, nor amount, nor number. He must offer a valid expiation for every oath he broke.