\[\boxed{\mathbf{7}\mathbf{.}}\]
\[\textbf{а)}\ 35 - 5 = 30\]
\[42 - 2 = 2\]
\[71 - 1 = 70\]
\[20 - 4 = 16\]
\[50 - 2 = 48\]
\[60 - 3 = 57\]
\[23 - 3 - 1 = 20 - 1 = 19\]
\[34 - 4 - 2 = 30 - 2 = 28\]
\[45 - 5 - 3 = 40 - 3 = 37\]
\[\textbf{б)}\ 76 - 6 - 7 = 70 - 7 = 63\]
\[88 - 8 - 6 = 80 - 6 = 74\]
\[94 - 4 - 9 = 90 - 9 = 81\]