\[\boxed{\mathbf{523.}}\]
\[\textbf{а)}\ 282 + 13 \cdot 3 = 282 + 39 =\]
\[= 321\]
\[520 - 68\ :2 = 520 - 34 = 486\]
\[240 \cdot 2\ :4 = 480\ :4 = 120\]
\[\textbf{б)}\ 786 + 428\ :2 =\]
\[= 786 + 214 = 1000\]
\[900 - 404 \cdot 2 = 900 - 808 = 92\]
\[936\ :3 - 295 = 312 - 295 = 17\]