\[\boxed{\mathbf{5.}}\]
\[\textbf{а)}\ 25 + 3 = 28\]
\[36 + 2 = 38\]
\[51 + 4 = 55\]
\[5 + 42 = 47\]
\[3 + 74 = 77\]
\[6 + 31 = 37\]
\[7 + 22 = 29\]
\[31 + 6 = 37\]
\[4 + 44 = 48\]
\[\textbf{б)}\ 40 + (11 - 9) = 40 + 2 = 42\]
\[(12 - 5) + 21 = 7 + 21 = 28\]
\[33 + (13 - 9) = 33 + 4 = 37\]