\[\boxed{\mathbf{7.}}\]
\[\textbf{а)}\ 27 + 7 = 34\]
\[45 - 6 = 39\]
\[3 + 38 = 41\]
\[70 - 62 = 8\]
\[52 - (20 + 12) = 52 - 32 = 20\]
\[37 + (33 - 30) = 37 + 3 = 40\]
\[63 - 9 + 45 = 54 + 45 = 99\]
\[50 - (55 - 6) = 50 - 49 = 1\]
\[\textbf{б)}\ 36 - (93 - 57) = 36 - 36 =\]
\[= 0\]
\[78 + (34 - 19) = 78 + 15 = 93\]
\[100 - (6 + 35) = 100 - 41 =\]
\[= 59\]
\[92 - (90 - 2) = 92 - 88 = 4\]