Tính một cách hợp lí

$\mathrm{1,2.}\frac{15}{4}+\frac{16}{7}.\frac{-85}{8}-\mathrm{1,2.5}\frac{3}{4}-\frac{16}{7}.\frac{-71}{8}$

$=\mathrm{1,2.}\frac{15}{4}-\mathrm{1,2.}\frac{23}{4}+\frac{16}{7}.\frac{-85}{8}-\frac{16}{7}.\frac{-71}{8}$

$=\mathrm{1,2.}\frac{15}{4}-\mathrm{1,2.}\frac{23}{4}+\frac{16}{7}.\frac{-85}{8}+\frac{16}{7}.\frac{71}{8}$

$=\mathrm{1,2.}\left(\frac{15}{4}-\frac{23}{4}\right)+\frac{16}{7}.\left(\frac{-85}{8}+\frac{71}{8}\right)$

$=\mathrm{1,2.}\frac{-8}{4}+\frac{16}{7}.\frac{-14}{8}$

$=\mathrm{1,2.}\left(-2\right)+\frac{16.\left(-14\right)}{7.8}$

$=-\mathrm{2,4}+\left(-4\right)$

$=-\mathrm{6,4}$