\[\boxed{\mathbf{Вариант\ 2.}}\]
\[\boxed{\mathbf{1.}}\]
\[\boxed{\mathbf{2.}}\]
\[1)\ 120\ :x = 400 - 340\]
\[120\ :x = 60\]
\[x = 120\ :60\]
\[x = 2.\]
\[Проверка:\]
\[120\ :2 = 400 - 340\]
\[60 = 60.\]
\[2)\ x\ :8 = 5 \cdot 16\]
\[x\ :8 = 80\]
\[x = 80 \cdot 8\]
\[x = 640.\]
\[Проверка:\]
\[640\ :8 = 5 \cdot 16\]
\[80 = 80.\]
\[\boxed{\mathbf{3.}}\]