21就twenty-one第一到第一百的英文單詞的簡寫,第一到第十：First，Second，Third，Fourth，Fifth，Sixth，Seventh，Eighth，Ninth，Tenth；第十一到第二十：Eleventh，Twelfth，Thirteenth,Fourteenth，Fifteenth，Sixteenth，Seventeenth，Eighteenth，Nineteenth，Twenty；第二十一到第二十九：Twenty-first/second/third/fourth/fifth...