Thực hiện các phép tính

$$\begin{gathered} a)1\frac{7}{20}÷2,7+2,7÷1.35+\left(0,4÷2\frac{1}{2}\right)\times \left(4.2-1\frac{3}{40}\right) \\ b)\left[\left(6\frac{3}{5}-3\frac{3}{14}\right)\times 5\frac{5}{6}\right]÷\left[\left(21-1.25\right)÷2,5\right] \\ c)\frac{-3}{5}\times \frac{5}{7}+\frac{-3}{5}\times \frac{3}{7}+\frac{-3}{5}\times \frac{6}{7} \\ d)\frac{4}{5}-\left(1\frac{2}{3}+3\frac{4}{5}\right) \end{gathered}$$