\[\boxed{\mathbf{Страница\ 43.}еуроки - ответы\ на\ пятёрку}\]
\[\boxed{\mathbf{2.1.}}\]
\[\textbf{а)}\]
\[\textbf{б)}\ A(3;4).\]
\[\boxed{\mathbf{2.2.}}\]
\[800 - (320\ :x - 25) \cdot 30 = 350\]
\[(320\ :x - 25) \cdot 30 = 800 - 350\]
\[(320\ :x - 25) \cdot 30 = 450\]
\[320\ :x - 25 = 450\ :30\]
\[320\ :x - 25 = 15\]
\[320\ :x = 15 + 25\]
\[320\ :x = 40\]
\[x = 320\ :40\]
\[x = 8.\]
\[\boxed{\mathbf{2.3.}}\]
\[\textbf{а)}\ x\ :4 - x\ :5\]
\[\textbf{б)}\ 8 \cdot (k + n) + b\]
\[\boxed{\mathbf{3.}}\]
\[300\ \frac{м}{мин} = 18\ \frac{км}{ч} - Игорь;\]
\[150\ \frac{м}{мин} = 9\ \frac{км}{ч} - Маша.\]
\[Ответ:C.12\frac{км}{ч}.\]