a) $\frac{12}{23}.\frac{7}{13}+\frac{11}{23}.\frac{7}{13} = \frac{7}{13}.(\frac{12}{23}+\frac{11}{23})=\frac{7}{13}$

b) $\frac{4}{9}.\frac{23}{11}−\frac{1}{11}.\frac{4}{9}+\frac{4}{9} = \frac{4}{9}.(\frac{23}{11}−\frac{1}{11}+1)=\frac{4}{9}.3=\frac{4}{3}$

c) $[(\frac{−5}{7})+\frac{3}{5}]:\frac{2020}{2021}+(\frac{2}{5}−\frac{2}{7}):\frac{2020}{2021}= [(\frac{−5}{7})+\frac{3}{5}+\frac{2}{5}+(\frac{−2}{7})]:\frac{2020}{2021}$=0

d) $\frac{3}{8}:(\frac{7}{22}−\frac{2}{11})+\frac{3}{8}:(\frac{2}{5}−\frac{1}{10})= \frac{3}{8}:\frac{3}{22}+\frac{3}{8}:\frac{3}{10}=\frac{3}{8}.(\frac{22}{3}+\frac{10}{3})$=4