\[Страница\ 20.\]
\[\boxed{\mathbf{4}.}\]
\[\textbf{а)}\ 3 + 2\]
\[\textbf{б)}\ 0 + 4\]
\[\textbf{в)}\ 5 + 0\]
\[\boxed{\mathbf{5}.}\]
\[\textbf{а)}\ 3 + 497 = 497 + 3 = 500\]
\[\textbf{б)}\ 5 + 2038 = 2038 + 5 = 2043\]
\[\boxed{\mathbf{6}.}\]
\[\textbf{а)}\ (184 + 239) + 61 = 184 +\]
\[+ (239 + 61) = 184 + 300 = 484\]
\[\textbf{б)}\ (500 + 342) + 158 = 500 +\]
\[+ (342 + 158) = 500 + 500 =\]
\[= 1000\]
\[\textbf{в)}\ 584 + (116 + 258) =\]
\[= (584 + 116) + 258 =\]
\[= 700 + 258 = 958\]
\[\textbf{г)}\ 726 + (274 + 318) =\]
\[= (726 + 274) + 318 =\]
\[= 1000 + 318 = 1318\]
\[\boxed{\mathbf{7}.}\]
\[\textbf{а)}\ 37 + 206 + 163 =\]
\[= (37 + 163) + 206 =\]
\[= 200 + 206 = 406\]
\[\textbf{б)}\ 621 + 257 + 143 + 79 =\]
\[= (621 + 79) + (257 + 143) =\]
\[= 700 + 400 = 1100\]
\[\boxed{\mathbf{8}.}\]
\[\textbf{а)}\ 90\ 046 = 9 \cdot 10\ 000 +\]
\[+ 4 \cdot 10 + 6 \cdot 1\]
\[\textbf{б)}\ 3\ 007\ 200 = 3 \cdot 1\ 000\ 000 +\]
\[+ 7 \cdot 1000 + 2 \cdot 100\]
\[\textbf{в)}\ 40\ 060\ 030 = 4 \cdot 10\ 000\ 000 +\]
\[+ 6 \cdot 10\ 000 + 3 \cdot 10\]
\[\textbf{г)}\ 5\ 700\ 800\ 000 =\]
\[= 5 \cdot 1\ 000\ 000\ 000 +\]
\[+ 7 \cdot 100\ 000\ 000 + 8 \cdot 100\ 000\]