\[\boxed{\mathbf{1.}}\]
\[\textbf{а)}\ 2\ 075\ 800 =\]
\[= 2\ 000\ 000 + 70\ 000 + 5000 + 800\]
\[\textbf{б)}\ 2\ 075\ 799\]
\[\boxed{\mathbf{2.}}\]
\[\textbf{а)}\ 60 + 24 = 84\]
\[\textbf{б)}\ 17 \cdot 3 = 51\]
\[\textbf{в)}\ 39 + 54\ :18 = 39 + 3 = 42\]
\[\boxed{\mathbf{3.}}\]
\[\boxed{\mathbf{4.}}\]
\[= 24 + 0 \cdot 9 - 56\ :56 + 4 \cdot 1 =\]
\[= 24 - 1 + 4 = 27\]
\[\boxed{\mathbf{5.}}\]
\[9 \cdot 7 = 63 - 3 = 60\ :5 =\]
\[= 12 + 4 = 16\]
\[Ответ:Вася\ задумал\ число\ 16.\]