\(9:81.9=90:819=\frac{90}{891}=\frac{90:9}{891:9}=\boxed{\frac{10}{99}}\\\\7.4:103.6=74:1036=\frac{74}{1036}=\frac{74:74}{1036:74}=\boxed{\frac{1}{14}}\\\\0.87:0.5307=8700:5307=\frac{8700}{5307}=1\frac{3393}{5307}=1\frac{3393:3}{5307:3}=1\frac{1131}{1769}\\\\=1\frac{1131:29}{1769:29}=\boxed{1\frac{39}{61}}\\\\5.5:3.25=550:325=\frac{550}{325}=\frac{550:25}{325:25}=\frac{22}{13}=\boxed{1\frac{9}{13}}\)