Đáp án:

$\begin{array}{l}
1)a)\dfrac{8}{{ - 15}} = \dfrac{{ - 8}}{{15}} = \dfrac{{ - 16}}{{30}}\\
\dfrac{{ - 5}}{6} = \dfrac{{ - 25}}{{30}} < \dfrac{{ - 16}}{{30}}\\
 \Leftrightarrow \dfrac{8}{{ - 15}} > \dfrac{{ - 5}}{6}\\
b)1\dfrac{3}{4} = \dfrac{7}{4}\\
 - 0,25 = \dfrac{{ - 1}}{4}\\
60\%  = \dfrac{{60}}{{100}} = \dfrac{3}{5}\\
B2)\\
a)\left( {\dfrac{{11}}{{12}} + 7} \right) + \left( { - 8 + \dfrac{1}{{12}}} \right)\\
 = \dfrac{{11}}{{12}} + \dfrac{1}{{12}} + 7 - 8\\
 = \dfrac{{12}}{{12}} - 1\\
 = 1 - 1\\
 = 0\\
b)\dfrac{4}{5}.19\dfrac{1}{3} - \dfrac{4}{5}.39\dfrac{1}{3}\\
 = \dfrac{4}{5}.\left( {19\dfrac{1}{3} - 39\dfrac{1}{3}} \right)\\
 = \dfrac{4}{5}.\left( {19 + \dfrac{1}{3} - 39 - \dfrac{1}{3}} \right)\\
 = \dfrac{4}{5}.\left( { - 20} \right)\\
 =  - 16\\
c)125\% .{\left( {\dfrac{{ - 1}}{2}} \right)^2}:\left( {1\dfrac{5}{{16}} - 1,5} \right) + {2021^0}\\
 = \dfrac{{125}}{{100}}.\dfrac{1}{4}:\left( {\dfrac{{21}}{{16}} - \dfrac{3}{2}} \right) + 1\\
 = \dfrac{5}{4}.\dfrac{1}{4}:\left( {\dfrac{{21}}{{16}} - \dfrac{{24}}{{16}}} \right) + 1\\
 = 5:\dfrac{{ - 3}}{{16}} + 1\\
 = 5.\dfrac{{ - 16}}{3} + 1\\
 = \dfrac{{ - 80}}{3} + 1\\
 = \dfrac{{ - 77}}{3}\\
B3)\\
a)\dfrac{1}{6} + x = \dfrac{2}{3}\\
 \Leftrightarrow x = \dfrac{2}{3} - \dfrac{1}{6}\\
 \Leftrightarrow x = \dfrac{4}{6} - \dfrac{1}{6} = \dfrac{3}{6} = \dfrac{1}{2}\\
Vậy\,x = \dfrac{1}{2}\\
b)1,5:x - \dfrac{5}{8} = \dfrac{7}{8}\\
 \Leftrightarrow \dfrac{3}{2}:x = \dfrac{7}{8} + \dfrac{5}{8}\\
 \Leftrightarrow \dfrac{3}{2}:x = \dfrac{{12}}{8} = \dfrac{3}{2}\\
 \Leftrightarrow x = \dfrac{3}{2}:\dfrac{3}{2} = 1\\
Vậy\,x = 1\\
c)\left( {\dfrac{{3x}}{7} + 1} \right):\left( { - 4} \right) = \dfrac{{ - 1}}{{28}}\\
 \Leftrightarrow \dfrac{{3x}}{7} + 1 = \dfrac{{ - 1}}{{28}}.\left( { - 4} \right)\\
 \Leftrightarrow \dfrac{{3x}}{7} + 1 = \dfrac{1}{7}\\
 \Leftrightarrow \dfrac{{3x}}{7} = \dfrac{1}{7} - 1 = \dfrac{{ - 6}}{7}\\
 \Leftrightarrow x = \dfrac{{ - 6}}{7}:\dfrac{3}{7}\\
 \Leftrightarrow x =  - 2\\
Vậy\,x =  - 2
\end{array}$