Đáp án:
$\begin{array}{l}
A = - \frac{1}{2} + \frac{5}{6} - 3,12 + 5.1\\
= - \frac{1}{2} + \frac{5}{6} - \frac{{312}}{{100}} + 5\\
= \frac{{ - 1}}{2} + \frac{5}{6} - \frac{{78}}{{25}} + 5\\
= \frac{{ - 1.75 + 5.25 - 78.6 + 5.150}}{{150}}\\
= \frac{{332}}{{150}} = \frac{{166}}{{75}}\\
B = \left( {\frac{4}{{15}} + \frac{7}{{12}} - \frac{9}{{20}}} \right).\frac{2}{5} + 0,25\\
= \frac{{4.4 + 7.5 - 9.3}}{{60}}.\frac{2}{5} + \frac{1}{4}\\
= \frac{{24}}{{30.5}} + \frac{1}{4}\\
= \frac{4}{{25}} + \frac{1}{4}\\
= \frac{{4.4 + 25}}{{100}} = \frac{{33}}{{100}}
\end{array}$
