四十：forty ['fɔ:ti]第四十：fortieth ['fɔ:tiiθ]
五十：fifty ['fifti]fiftieth ['fiftiiθ]
六十：sixty ['siksti] sixtieth ['sikstiiθ]
七十：seventy ['sevənti]seventieth ['sevəntiiθ]
八十：eighty ['eiti]eightieth ['eitiiθ]
九十：ninety ['nainti]ninetieth ['naintiiθ]
后面的都是序数词,前面都是基数词