\[\boxed{\text{742\ (742).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \overline{\text{xy}} = 10x + y\]
\[\textbf{б)}\ \overline{\text{yx}} = 10y + x\]
\[\textbf{в)}\ \overline{a0b} = 100a + 10 \cdot 0 + b =\]
\[= 100a + b\]
\[\textbf{г)}\ \ \overline{\text{abcd}} =\]
\[= 1000a + 100b + 10c + d\]