Đáp án:

 $a) \dfrac{2}{3}-\dfrac{1}{5}:\dfrac{2}{25}+\dfrac{-1}{6}$

$= \dfrac{2}{3}-(\dfrac{1}{5}.\dfrac{25}{2})+\dfrac{-1}{6}$

$= \dfrac{2}{3}- \dfrac{5}{2}+\dfrac{-1}{6}$

$= \dfrac{4}{6}-\dfrac{15}{6}+\dfrac{-1}{6}$

$= \dfrac{4-15+(-1)}{6}$

$= \dfrac{-12}{6}$

$= -2$

$b) \dfrac{-4}{5}.\dfrac{7}{8}-\dfrac{7}{8}$

$= (\dfrac{(-4).7}{5.8})-\dfrac{7}{8}$

$= \dfrac{-7}{10}-\dfrac{7}{8}$

$= \dfrac{-28}{40}-\dfrac{35}{40}$

$= \dfrac{(-28)-35}{40}$

$= \dfrac{-63}{40}$

$c) \dfrac{1}{10}+\dfrac{5}{12}:\dfrac{1}{-6}$

$= \dfrac{1}{10}+(\dfrac{5}{12}.\dfrac{-6}{1})$

$= \dfrac{1}{10}+\dfrac{-5}{2}$

$= \dfrac{1}{10}+\dfrac{-25}{10}$

$= \dfrac{1+(-25)}{10}$

$= \dfrac{-24}{10}$

$= \dfrac{-12}{5}$

$d) \dfrac{-8}{3}.\dfrac{1}{40}-\dfrac{1}{15}$

$= \dfrac{(-2)^3.1}{3.2^3.5}- \dfrac{1}{15}$

 $= \dfrac{-1}{15}-\dfrac{1}{15}$

$= \dfrac{(-1)-1}{15}$

$= \dfrac{-2}{15}$

$\text{a) $\dfrac{2}{3}$ - $\dfrac{1}{5}$ : $\dfrac{2}{25}$ + $\dfrac{-1}{6}$}$

$\text{= $\dfrac{2}{3}$ - $\dfrac{1}{5}$ . $\dfrac{25}{2}$ + $\dfrac{-1}{6}$}$

$\text{= $\dfrac{2}{3}$ - $\dfrac{5}{2}$ + $\dfrac{-1}{6}$}$

$\text{= $\dfrac{8}{12}$ - $\dfrac{30}{12}$ + $\dfrac{-2}{12}$}$

$\text{= $\dfrac{12}{12}$ + $\dfrac{-2}{12}$}$

$\text{= $\dfrac{10}{12}$ = $\dfrac{5}{6}$}$

$\text{b) $\dfrac{-4}{5}$ . $\dfrac{7}{8}$ - $\dfrac{7}{8}$}$

$\text{= $\dfrac{-28}{40}$ - $\dfrac{7}{8}$}$

$\text{= $\dfrac{-28}{40}$ - $\dfrac{35}{40}$}$

$\text{= $\dfrac{-63}{40}$ }$

$\text{c) $\dfrac{1}{10}$ + $\dfrac{5}{12}$ : $\dfrac{1}{-6}$ }$

$\text{= $\dfrac{1}{10}$ + $\dfrac{5}{12}$ . $\dfrac{-6}{1}$ }$

$\text{= $\dfrac{1}{10}$ + $\dfrac{-30}{12}$ }$

$\text{= $\dfrac{6}{60}$ + $\dfrac{-150}{60}$ }$

$\text{= $\dfrac{144}{60}$ = $\dfrac{12}{5}$}$

$\text{d) $\dfrac{-8}{3}$ . $\dfrac{1}{40}$ - $\dfrac{1}{15}$ }$

$\text{= $\dfrac{-8}{120}$ - $\dfrac{1}{15}$ }$

$\text{= $\dfrac{-4}{60}$ - $\dfrac{4}{60}$ }$

$\text{= $\dfrac{-8}{60}$ = $\dfrac{-2}{15}$}$