a) Ta có: $\frac{-11}{-30} = \frac{11}{30}$.

• $\frac{3}{20} = \frac{3\times 3}{20\times 3} = \frac{9}{60}$;
• $\frac{11}{30} = \frac{11\times 2}{30\times 2} = \frac{22}{60}$;
• $\frac{-7}{15} = \frac{-7\times 4}{15\times 4} = \frac{-28}{60}$;

b) Ta có: $\frac{27}{-180} = \frac{-3}{20}$.

• $\frac{3}{35} = \frac{3\times 3}{20\times 3} = \frac{9}{140}$;
• $\frac{27}{-180} = \frac{-3}{20} = \frac{-3\times 7}{20\times 7} = \frac{-21}{140}$;
• $\frac{-3}{-28} = \frac{3}{28} = \frac{3\times 5}{28\times 5} = \frac{15}{140}$;