\[\boxed{\mathbf{2.}}\]
\[Сначала\ выполняем\ \]
\[умножение\ или\ деление,\ затем\ \]
\[сложение\ или\ вычитание.\]
\[\textbf{а)}\ 20 + 2 \cdot 5 = 20 + 10 = 30\]
\[30 + 2 \cdot 7 = 30 + 14 = 44\]
\[20 - 2 \cdot 6 = 20 - 12 = 8\]
\[30 - 2 \cdot 4 = 30 - 8 = 22\]
\[40 + 4\ :2 = 40 + 2 = 42\]
\[50 + 8\ :2 = 50 + 4 = 54\]
\[35 - 10\ :2 = 35 - 5 = 30\]
\[26 - 12\ :2 = 26 - 6 = 20\]
\[\textbf{б)}\ 55 + 3 \cdot 5 = 55 + 15 = 70\]
\[5 \cdot 4 - 18 = 20 - 18 = 2\]
\[43 + 14\ :2 = 43 + 7 = 50\]
\[80 - 18\ :3 = 80 - 6 = 74\]