Thực hiện phép tính (tính hợp lí nếu có thể):  a)(- 0,4).(- 2,5).(- 0,8); b)5/- 8+ 7/8.2/3

a) \(\left( { - 0,4} \right).\left( { - 2,5} \right).\left( { - 0,8} \right)\)

\( = 1.\left( { - 0,8} \right)\)\( =  - 0,8\).

c) \({\left( { - 2} \right)^3}.\frac{{ - 1}}{{24}} + \left( {\frac{4}{5} - 1,2} \right):\frac{2}{{15}}\)

\( = \left( { - 8} \right).\frac{{ - 1}}{{24}} + \left( {\frac{4}{5} - \frac{6}{5}} \right).\frac{{15}}{2}\)

\( = \frac{1}{3} + \frac{{ - 2}}{5}.\frac{{15}}{2}\)

\( = \frac{1}{3} + \left( { - 3} \right)\)

\( = \frac{{ - 8}}{3}\).

b) \(\frac{5}{{ - 8}} + \frac{7}{8}.\frac{2}{3}\)\( = \frac{{ - 5}}{8} + \frac{7}{{12}}\)

\( = \frac{{ - 15}}{{24}} + \frac{{14}}{{24}}\)\( =  - \frac{1}{{24}}\).

d) \(\frac{{ - 15}}{{14}}:\frac{{17}}{{23}} - \frac{{15}}{{14}}:\frac{{17}}{{11}} - \frac{6}{7}\)

\( = \frac{{ - 15}}{{14}}.\frac{{23}}{{17}} - \frac{{15}}{{14}}.\frac{{11}}{{17}} - \frac{6}{7}\)

\( = \frac{{ - 15}}{{14}}.\left( {\frac{{23}}{{17}} + \frac{{11}}{{17}}} \right) - \frac{6}{7}\)

\( = \frac{{ - 15}}{{14}}.2 - \frac{6}{7}\)

\( = \frac{{ - 15}}{7} - \frac{6}{7}\)\( = \frac{{ - 21}}{7} =  - 3\).