b:c=0.5:2/3=>b:c=1/2:2/3,同乘6=>b:c=3:4a:b=4:5=>a/b=4/5,右边分子分母同乘3=>a/b=12/15b:c=3:4=>b/c=3/4,右边分子分母同乘5=>b/c=15/20所以,a:b:c=12:15:20
