b)=$\frac{20:5}{-25:5}$=$\frac{4}{-5}$=$\frac{4.(-1)}{-5.(-1)}$=$\frac{-4}{5}$
c)$\frac{-20}{50}$=$\frac{-20:2}{50:2}$=$\frac{-10}{25}$=$\frac{-10:5}{25:5}$=$\frac{-2}{5}$
d)=$\frac{105:5}{225:5}$=$\frac{21}{45}$=$\frac{21:3}{45:3}$=$\frac{7}{15}$
e)$\frac{210}{1680}$=$\frac{210:2}{1680:2}$ =$\frac{105}{840}$=$\frac{105:5}{840:5}$
=$\frac{21}{168}$=$\frac{21:3}{168:3}$=$\frac{7}{56}$=$\frac{1}{7}$
