\[\boxed{\mathbf{Вариант\ 2.}}\]
\[\boxed{\mathbf{1.}}\]

\[\boxed{\mathbf{2.}}\]

\[Проверка:\]
\[78 \cdot 30 + 20 = 2340 + 20 = 2360\]

\[87 \cdot 800 + 100 = 69\ 600 + 100 =\]
\[= 69\ 700\]

\[\boxed{\mathbf{3.}}\]
\[80 \cdot 20 - 600 = 1000\]
\[900 + 20 \cdot 15 = 1200\]