Đáp án:
$\begin{array}{l}
21)49\dfrac{8}{{23}} - \left( {5\dfrac{7}{{32}} + 14\dfrac{8}{{23}}} \right)\\
= 49 + \dfrac{8}{{23}} - 5 - \dfrac{7}{{32}} - 14 - \dfrac{8}{{23}}\\
= 49 - 5 - 14 - \dfrac{7}{{32}}\\
= 30 - \dfrac{7}{{32}}\\
= \dfrac{{913}}{{32}}\\
22)\dfrac{{ - 3}}{7}.\dfrac{5}{9} + \dfrac{4}{9}.\dfrac{{ - 3}}{7} + 2\dfrac{3}{7}\\
= \dfrac{{ - 3}}{7}\left( {\dfrac{5}{9} + \dfrac{4}{9}} \right) + 2 + \dfrac{3}{7}\\
= \dfrac{{ - 3}}{7}.1 + 2 + \dfrac{3}{7}\\
= 2\\
23)8\dfrac{2}{7} - \left( {3\dfrac{4}{9} + 4\dfrac{2}{7}} \right)\\
= 8 + \dfrac{2}{7} - 3 - \dfrac{4}{9} - 4 - \dfrac{2}{7}\\
= 8 - 3 - 4 - \dfrac{4}{9}\\
= 1 - \dfrac{4}{9}\\
= \dfrac{5}{9}\\
24)\left[ {\left( {6\dfrac{3}{5} - 3\dfrac{3}{{14}}} \right).5\dfrac{5}{6}} \right]:\left[ {\left( {21 - 1,25} \right):2,5} \right]\\
= \left( {\dfrac{{33}}{5} - \dfrac{{45}}{{14}}} \right).\dfrac{{35}}{6}:\left( {19,75:2,5} \right)\\
= \dfrac{{237}}{{70}}.\dfrac{{35}}{6}:7,9\\
= \dfrac{{237}}{{12}}.\dfrac{{10}}{{79}}\\
= \dfrac{{3.5}}{6}\\
= \dfrac{5}{2}
\end{array}$
