Đáp án:

$\begin{array}{l}
B1)\\
a)1,2 - x - 0,8 =  - 20,9 + x\\
 \Rightarrow x + x = 1,2 - 0,8 + 20,9\\
 \Rightarrow 2x = 0,4 + 20,9\\
 \Rightarrow 2x = 21,3\\
 \Rightarrow x = 10,65\\
b)2,3x + 20,7 + 2x = 3,6 - 1,7x\\
 \Rightarrow 2,3x + 2x + 1,7x = 3,6 - 20,7\\
 \Rightarrow 6x =  - 17,1\\
 \Rightarrow x =  - 17,1:6 =  - \dfrac{{57}}{{20}}\\
c)4x - 5 = 2 + 7\\
 \Rightarrow 4x = 9 + 5\\
 \Rightarrow 4x = 14\\
 \Rightarrow x = \dfrac{7}{2}\\
B2)a)x - \dfrac{3}{5} = 6 - \dfrac{{1 - 2x}}{3}\\
 \Rightarrow x - \dfrac{3}{5} = 6 - \dfrac{1}{3} + \dfrac{2}{3}x\\
 \Rightarrow x - \dfrac{2}{3}x = 6 - \dfrac{1}{3} + \dfrac{3}{5}\\
 \Rightarrow \dfrac{1}{3}x = \dfrac{{94}}{{15}}\\
 \Rightarrow x = \dfrac{{94}}{5}\\
b)\dfrac{{3x - 2}}{6} - 5 = 3 - \dfrac{{2x + 7}}{4}\\
 \Rightarrow \dfrac{x}{2} - \dfrac{1}{3} - 5 - 3 + \dfrac{x}{2} + \dfrac{7}{4} = 0\\
 \Rightarrow x - \dfrac{{79}}{{12}} = 0\\
 \Rightarrow x = \dfrac{{79}}{{12}}\\
c)2\left( {x + \dfrac{3}{5}} \right) = 5 - \left( {\dfrac{{13}}{5} + x} \right)\\
 \Rightarrow 2x + \dfrac{6}{5} = 5 - \dfrac{{13}}{5} - x\\
 \Rightarrow 2x + x = 5 - \dfrac{{13}}{5} - \dfrac{6}{5}\\
 \Rightarrow 3x = \dfrac{6}{5}\\
 \Rightarrow x = \dfrac{2}{5}\\
d)10x - 12 - 3x = 6 + x\\
 \Rightarrow 10x - 3x - x = 6 + 12\\
 \Rightarrow 6x = 18\\
 \Rightarrow x = 3\\
e)\dfrac{{7x - 1}}{6} + 2x = \dfrac{{16 - x}}{5}\\
 \Rightarrow \dfrac{{7x}}{6} - \dfrac{1}{6} + 2x = \dfrac{{16}}{5} - \dfrac{x}{5}\\
 \Rightarrow \left( {\dfrac{{7x}}{6} + 2x + \dfrac{x}{5}} \right) = \dfrac{{16}}{5} + \dfrac{1}{6}\\
 \Rightarrow \dfrac{{101}}{{30}}x = \dfrac{{101}}{{30}}\\
 \Rightarrow x = 1\\
f)\dfrac{{3x - 1}}{4} + \dfrac{{6x - 2}}{8} = \dfrac{{1 - 3x}}{6}\\
 \Rightarrow \dfrac{{3x - 1}}{4} + \dfrac{{3x - 1}}{4} = \dfrac{{1 - 3x}}{6}\\
 \Rightarrow \dfrac{{3x - 1}}{2} = \dfrac{{1 - 3x}}{6}\\
 \Rightarrow 3\left( {3x - 1} \right) = 1 - 3x\\
 \Rightarrow 9x - 3 = 1 - 3x\\
 \Rightarrow 9x + 3x = 1 + 3\\
 \Rightarrow 12x = 4\\
 \Rightarrow x = \dfrac{1}{3}\\
g)\dfrac{{5x - 4}}{3} = \dfrac{{2 + 3x}}{2}\\
 \Rightarrow 2\left( {5x - 4} \right) = 3\left( {2 + 3x} \right)\\
 \Rightarrow 10x - 8 = 6 + 9x
\end{array}$

$ \Rightarrow x = 14$

Vậy x=14