20/45,分子分母同除以5.26*2=52,65-52=13,13/26=0.5=1/2,所以，65/26=2又2分之1.84里头才有4个21,81不够21的4倍。仅仅是3倍多。26/13=2,78/13=6,26/78=1/3.
20/45=4/965/26=5/2=2½81/21=27/7=3又6/726/78=13/39=⅓
