Đáp án:
$\begin{array}{l}
a)\left( {\dfrac{1}{2} + \dfrac{3}{5} - \dfrac{1}{3}} \right).0,8 + 0,5.\left( { - 2\dfrac{1}{2}} \right):1\dfrac{1}{4}\\
= \dfrac{{15 + 3.6 - 10}}{{30}}.\dfrac{8}{{10}} + \dfrac{5}{{10}}.\left( { - \dfrac{5}{2}} \right):\dfrac{5}{4}\\
= \dfrac{{23}}{{30}}.\dfrac{4}{5} + \dfrac{1}{2}.\left( { - \dfrac{5}{2}} \right).\dfrac{4}{5}\\
= \dfrac{{46}}{{75}} - 1\\
= \dfrac{{ - 29}}{{75}}\\
b)\dfrac{1}{4}.13\dfrac{9}{{11}} - 0,25.6\dfrac{2}{{11}}\\
= \dfrac{1}{4}.\dfrac{{152}}{{11}} - \dfrac{{25}}{{100}}.\dfrac{{68}}{{11}}\\
= \dfrac{1}{4}.\dfrac{{152}}{{11}} - \dfrac{1}{4}.\dfrac{{68}}{{11}}\\
= \dfrac{1}{4}.\left( {\dfrac{{152}}{{11}} - \dfrac{{68}}{{11}}} \right)\\
= \dfrac{1}{4}.\dfrac{{84}}{{11}}\\
= \dfrac{{21}}{{11}}
\end{array}$
