Đáp án:
$\begin{array}{l}
7)b)1\dfrac{2}{3}.x - \dfrac{4}{5} = 2\dfrac{1}{5}\\
\Rightarrow \dfrac{5}{3}.x = \dfrac{4}{5} + \dfrac{{11}}{5}\\
\Rightarrow \dfrac{5}{3}.x = \dfrac{{15}}{5} = 3\\
\Rightarrow x = 3.\dfrac{3}{5}\\
\Rightarrow x = \dfrac{9}{5}\\
Vậy\,x = \dfrac{9}{5}
\end{array}$