\[\boxed{\mathbf{13.}}\]
\[\textbf{а)}\ 40 + 3 = 43\]
\[62 - 2 = 60\]
\[34 - 30 = 4\]
\[57 + 10 - 7 = 67 - 7 = 60\]
\[83 - 10 - 3 = 73 - 3 = 70\]
\[92 - 2 + 10 = 90 + 10 = 100\]
\[\textbf{б)}\ 39 + (90 - 80) = 39 + 10 =\]
\[= 49\]
\[73 - (70 - 60) = 73 - 10 = 63\]
\[100 - (45 - 5) = 100 - 40 =\]
\[= 60\]