先约分在再通分后比大小,从大到小15/35,30/63,12/20,30/45,35/75

答错，抱歉~

15/35约去5 =3/730/63约去3 =10/2112/20约去4 =3/530/45约去15 =2/335/75约去5 =7/15通分7,21,5,3,15 的最小公倍数为1053/7=45/10510/21=50/1053/5=63/1052/3=70/1057/15=49/105所以30/45＞12/20＞30/63＞35/75＞15/...