[tex]b) \ 0,8:23 = \frac{8}{10}:23 = \frac{4}{5}\cdot\frac{1}{23} = \frac{4}{115}\\\\c) \ 42 + (-3)(-2) = 42 + 6 = 48\\\\d) \ 21\cdot0,64:(-15) = 21\cdot\frac{64}{100}:(-15) = 21\cdot\frac{16}{25}\cdot(-\frac{1}{15}) =3\cdot\frac{16}{25}\cdot(-\frac{1}{5}) = -\frac{112}{125}[/tex]