a. 

$(\frac{6}{11}+\frac{5}{11})\times \frac{3}{7}= 1\times \frac{3}{7}=\frac{3}{7}$

$(\frac{6}{11}+\frac{5}{11})\times \frac{3}{7}= \frac{6}{11}\times\frac{3}{7} +\frac{5}{11}\times \frac{3}{7}=\frac{18}{77}+\frac{15}{77}=\frac{33}{77}=\frac{3}{7}$

b. 

$\frac{3}{5}\times \frac{7}{9}+\frac{3}{5}\times \frac{2}{9}=\frac{21}{45}+\frac{6}{45}=\frac{27}{45}=\frac{3}{5}$

$\frac{3}{5}\times \frac{7}{9}+\frac{3}{5}\times \frac{2}{9}=\frac{3}{5}\times (\frac{7}{9}+\frac{2}{9})=\frac{3}{5}\times 1=\frac{3}{5}$

c. 

$(\frac{6}{7}-\frac{4}{7}):\frac{2}{5}=\frac{2}{7}\times \frac{5}{2}=\frac{10}{14}$

$(\frac{6}{7}-\frac{4}{7}):\frac{2}{5}=\frac{6}{7}:\frac{2}{5}-\frac{4}{7}:\frac{2}{5}=\frac{6}{7}\times \frac{5}{2}-\frac{4}{7}\times \frac{5}{2}= \frac{30}{14}-\frac{20}{14}=\frac{10}{14}$

d. 

$\frac{8}{15}:\frac{2}{11}+\frac{7}{15}:\frac{2}{11}=\frac{8}{15}\times \frac{11}{2}+\frac{7}{15}\times \frac{11}{2}=\frac{88}{30}+\frac{77}{30}=\frac{165}{30}=\frac{11}{2}$

$\frac{8}{15}:\frac{2}{11}+\frac{7}{15}:\frac{2}{11}= (\frac{8}{15}+\frac{7}{15}):\frac{2}{11}= 1\times \frac{11}{2}=\frac{11}{2}$