（1）21:105 = 1:5 （2）7.2:0.42 = 720:42 = 120:7（3）5\6:4\9 =15/18:8/18 = 15:8(4)0.08:2 = 8:200=1:25(5)19\3:76 = 1/3:4 = 1:12(6)3.75:0.625=(3.75*8):(0.625*8)=30:5 = 6:1