\(\dfrac{21}{10}:\left(\dfrac{12}{5}-\dfrac{9}{10}\right)\left(-\dfrac{4}{7}\right)-\dfrac{3}{2}\left(\dfrac{1}{6}-\dfrac{7}{12}\right)+\dfrac{1}{5}\left(\dfrac{3}{2}-\dfrac{1}{4}\right)\)
\(=\dfrac{21}{10}:\left(\dfrac{24}{10}-\dfrac{9}{10}\right)\left(-\dfrac{4}{7}\right)-\dfrac{3}{2}\left(\dfrac{2}{12}-\dfrac{7}{12}\right)+\dfrac{1}{5}\left(\dfrac{6}{4}-\dfrac{1}{4}\right)\)
\(=\dfrac{21}{10}:\dfrac{3}{2}\left(-\dfrac{4}{7}\right)-\dfrac{3}{2}.\left(\dfrac{-5}{12}\right)+\dfrac{1}{5}.\dfrac{5}{4}\)
\(=\dfrac{7}{5}.\left(-\dfrac{4}{7}\right)+\dfrac{5}{8}+\dfrac{1}{4}\)
\(=-\dfrac{4}{5}+\dfrac{7}{8}=\dfrac{3}{40}\)
