Đáp án:

 $1)
a) \dfrac{-68}{483}\\
b)\dfrac{-14}{75}\\
c)
3\\
d)
\dfrac{-13}{18}\\
2)
a)x=\dfrac{7}{25}\\
b)
 x=\dfrac{21}{2}\\
c)
 {\left[\begin{aligned}x=\dfrac{5}{3}\\x=\dfrac{5}{4}\end{aligned}\right.}\\
d)
{\left[\begin{aligned}x=\dfrac{24}{13}\\x =\dfrac{14}{25}\end{aligned}\right.}\\$

Giải thích các bước giải:

 $1)
a) (-0,25).\dfrac{4}{7}.\left ( -3\dfrac{5}{21} \right ).\left ( -\dfrac{7}{23} \right )\\
=\dfrac{-1}{4}.\dfrac{4}{7}.\dfrac{-68}{21}.\dfrac{-7}{23}\\
=-\dfrac{1}{7}.\dfrac{-7}{23}.\dfrac{-68}{21}\\
=\dfrac{-68}{483}\\
b) \left ( -\dfrac{2}{5} \right ).\dfrac{4}{15}+\dfrac{-3}{10}.\dfrac{4}{15}\\
=\dfrac{4}{15}.\left ( \dfrac{-2}{5}-\dfrac{3}{10} \right )\\
=\dfrac{4}{15}.\left ( \dfrac{-4}{10}-\dfrac{3}{10} \right )\\
=\dfrac{4}{15}.\dfrac{-7}{10}\\
=\dfrac{-14}{75}\\
c)
21-3\dfrac{3}{4}:\left ( \dfrac{3}{8}-\dfrac{1}{6} \right )\\
=21-3\dfrac{3}{4}:\left ( \dfrac{9}{24}-\dfrac{4}{24} \right )\\
=21-\dfrac{15}{4}:\dfrac{5}{24}\\
=21-\dfrac{15}{4}.\dfrac{24}{5}\\
=21-18\\
=3\\
d)
\left ( -\dfrac{5}{6}+\dfrac{2}{5} \right ):\dfrac{3}{8}+\left ( \dfrac{4}{5}-\dfrac{11}{30} \right )\\
=\left ( -\dfrac{25}{30}+\dfrac{12}{30} \right ):\dfrac{3}{8}+\left ( \dfrac{24}{30}-\dfrac{11}{30} \right )\\
=\dfrac{-13}{30}.\dfrac{8}{3}+\dfrac{13}{30}\\
=\dfrac{13}{30}.\left ( \dfrac{-8}{3}+1 \right )\\
=\dfrac{13}{30}.\left ( \dfrac{-8}{3}+\dfrac{3}{3} \right )\\
=\dfrac{13}{30}.\dfrac{-5}{3}\\
=\dfrac{-13}{18}\\
2)
a) \dfrac{-2}{5}+\dfrac{5}{2}x=\dfrac{3}{10}\\
\Leftrightarrow \dfrac{5}{2}x=\dfrac{3}{10}+\dfrac{2}{5}\\
\Leftrightarrow \dfrac{5}{2}x=\dfrac{3}{10}+\dfrac{4}{10}\\
\Leftrightarrow \dfrac{5}{2}x=\dfrac{7}{10}\\
\Leftrightarrow x=\dfrac{7}{10}.\dfrac{2}{5}=\dfrac{7}{25}\\
b)
\dfrac{2}{3}+\dfrac{7}{4}:x=\dfrac{5}{6}\\
\Leftrightarrow \dfrac{7}{4}:x=\dfrac{5}{6}-\dfrac{2}{3}\\
\Leftrightarrow \dfrac{7}{4}:x=\dfrac{5}{6}-\dfrac{4}{6}\\
\Leftrightarrow \dfrac{7}{4}.\dfrac{1}{x}=\dfrac{1}{6}\\
\Leftrightarrow \dfrac{1}{x}=\dfrac{1}{6}.\dfrac{4}{7}=\dfrac{2}{21}\\
\Leftrightarrow x=\dfrac{21}{2}\\
c)
\left ( -x+\dfrac{5}{3} \right ).\left ( x-\dfrac{5}{4} \right )=0\\
\Leftrightarrow {\left[\begin{aligned}-x+\dfrac{5}{3}=0\\x-\dfrac{5}{4}=0\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{5}{3}\\x=\dfrac{5}{4}\end{aligned}\right.}\\
d)
\left (\dfrac{1}{3}x-\dfrac{8}{13}  \right ).\left ( 2,5+\dfrac{-7}{5}:x \right )=0\\
\Leftrightarrow {\left[\begin{aligned}\dfrac{1}{3}x-\dfrac{8}{13}=0\\2,5+\dfrac{-7}{5}:x =0\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}\dfrac{1}{3}x=\dfrac{8}{13}\\\dfrac{7}{5}:x =\dfrac{5}{2}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{24}{13}\\\dfrac{1}{x} =\dfrac{5}{2}.\dfrac{5}{7}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{24}{13}\\\dfrac{1}{x} =\dfrac{25}{14}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{24}{13}\\x =\dfrac{14}{25}\end{aligned}\right.}\\$