\(a,MSC:45\\ \dfrac{1}{5}=\dfrac{1\times9}{5\times9}=\dfrac{9}{45}\\ \dfrac{4}{9}=\dfrac{4\times5}{9\times5}=\dfrac{20}{45}\\ \Rightarrow\dfrac{9}{45}< \dfrac{20}{45}\Rightarrow\dfrac{1}{5}< \dfrac{4}{9}\)

\(b,\dfrac{-2020}{-2021}=\dfrac{2020}{2021}\\ \Rightarrow\dfrac{2020}{2019}>\dfrac{2020}{2021}\Rightarrow\dfrac{2020}{2019}>\dfrac{-2020}{-2021}\)

\(4-x=2\dfrac{3}{8}\\ \Rightarrow4-x=\dfrac{19}{8}\\ \Rightarrow\dfrac{32}{8}-x=\dfrac{19}{8}\\ \Rightarrow x=\dfrac{32}{8}-\dfrac{19}{8}\\ \Rightarrow x=\dfrac{13}{8}\)

\(35,7\times2,9=103,53\)

\(a\times5:6=25\\ \Rightarrow a\times5=25\times6\\ \Rightarrow a\times5=150\\ \Rightarrow a=150:5\\ \Rightarrow a=30\)

\(\left(\dfrac{-7}{5}+\dfrac{-3}{4}\right)\times\left(\dfrac{1}{2}+\dfrac{4}{5}\right)\\ =\left(\dfrac{-28}{20}+\dfrac{-15}{20}\right)\times\left(\dfrac{5}{10}+\dfrac{8}{10}\right)\\ =\dfrac{-43}{20}\times\dfrac{13}{10}\\ =\dfrac{-559}{200}\)

\(1+1=2\)

\(\left(\dfrac{2}{5}-x\right)^3\times50\%=\dfrac{1}{2}\\ \Rightarrow\left(\dfrac{2}{5}-x\right)^3\times\dfrac{1}{2}=\dfrac{1}{2}\\ \Rightarrow\left(\dfrac{2}{5}-x\right)^3=\dfrac{1}{2}:\dfrac{1}{2}\\ \Rightarrow\left(\dfrac{2}{5}-x\right)^3=\dfrac{1}{2}\times2\\ \Rightarrow\left(\dfrac{2}{5}-x\right)^3=1\\ \Rightarrow\dfrac{2}{5}-x=1\\ \Rightarrow x=\dfrac{2}{5}-1\\ \Rightarrow x=\dfrac{2}{5}-\dfrac{5}{5}\\ \Rightarrow x=\dfrac{-3}{5}\)