\[\boxed{\mathbf{489.}}\]
\[\textbf{а)}\ 395 + 46\ :2 = 392 + 23 =\]
\[= 415\]
\[524 - 24 \cdot 2 = 524 - 48 = 476\]
\[900\ :3 - 215 = 300 - 215 = 85\]
\[\textbf{б)}\ 1000 - 82\ :2 = 1000 - 41 =\]
\[= 959\]
\[1000 - 70 \cdot 4 = 1000 - 280 =\]
\[= 720\]
\[1000 - 420\ :6 = 1000 - 70 =\]
\[= 930\]