31的序數詞是31st,從123開始給你講一講,一序數詞是first,2的敍述詞是second,3的序數詞是3rd,從4開始都是直接在後面加th就可以。所以説21 31就是20,first,31st,41st。以此類推就可以,22 32 42就是22nd,32nd,42nd。