Đáp án:
$\begin{array}{l}
a)\dfrac{1}{7} - \dfrac{8}{7}:8 - 3:\dfrac{3}{4}.4\\
= \dfrac{1}{7} - \dfrac{8}{7}.\dfrac{1}{8} - 3.\dfrac{4}{3}.4\\
= \dfrac{1}{7} - \dfrac{1}{7} - 16\\
= - 16\\
c)1,4.\dfrac{{15}}{{49}} - \left( {\dfrac{4}{5} + \dfrac{2}{3}} \right):\dfrac{{11}}{5}\\
= \dfrac{7}{5}.\dfrac{{15}}{{49}} - \dfrac{{12 + 10}}{{15}}.\dfrac{5}{{11}}\\
= \dfrac{3}{7} - \dfrac{{22}}{{15}}.\dfrac{5}{{11}}\\
= \dfrac{3}{7} - \dfrac{2}{3}\\
= \dfrac{{9 - 14}}{{21}}\\
= \dfrac{{ - 5}}{{21}}\\
B2)\\
\dfrac{2}{3} < \dfrac{x}{{18}} < \dfrac{8}{9}\left( {x \in Z} \right)\\
\Leftrightarrow \dfrac{{2.6}}{{18}} < \dfrac{x}{{18}} < \dfrac{{8.2}}{{18}}\\
\Leftrightarrow \dfrac{{12}}{{18}} < \dfrac{x}{{18}} < \dfrac{{16}}{{18}}\\
\Leftrightarrow 12 < x < 16\\
\Leftrightarrow x \in \left\{ {13;14;15} \right\}\\
B3)\\
a:b = 1:\dfrac{3}{4}\\
\Leftrightarrow a:b = \dfrac{4}{3}\\
\Leftrightarrow \dfrac{a}{b} = \dfrac{4}{3}\\
\Leftrightarrow a = \dfrac{4}{3}.b\\
a + b = 22\\
\Leftrightarrow \dfrac{4}{3}.b + b = 22\\
\Leftrightarrow b.\left( {\dfrac{4}{3} + 1} \right) = 22\\
\Leftrightarrow b.\dfrac{7}{3} = 22\\
\Leftrightarrow b = 22.\dfrac{3}{7} = \dfrac{{66}}{7}\\
\Leftrightarrow a = \dfrac{4}{3}.b = \dfrac{4}{3}.\dfrac{{66}}{7} = \dfrac{{88}}{7}\\
Vậy\,a = \dfrac{{88}}{7};b = \dfrac{{66}}{7}
\end{array}$
