\[\boxed{\mathbf{251.}}\]
\[\textbf{а)}\ 450 + 330 = 780\]
\[265 + 520 = 785\]
\[665 + 222 = 887\]
\[560 - 320 = 240\]
\[344 - 230 = 114\]
\[438 - 125 = 313\]
\[\textbf{б)}\ 203 + 301 - 2 = 504 - 2 =\]
\[= 502\]
\[543 - 120 + 15 = 423 + 15 =\]
\[= 438\]
\[340 + 405 - 40 = 745 - 40 =\]
\[= 705\]