\[\boxed{\mathbf{5.}}\]
\[\textbf{а)}\ 31 + 17 = 48\]
\[26 + 21 = 47\]
\[42 + 32 = 74\]
\[53 + 25 = 78\]
\[45 + 12 = 57\]
\[14 + 73 = 87\]
\[\textbf{б)}\ 26 + 12 + 1 = 38 + 1 = 39\]
\[34 + 25 + 1 = 59 + 1 = 60\]
\[41 + 38 + 1 = 79 + 1 = 80\]
\[15 + 22 + 10 = 37 + 10 = 47\]
\[44 + 31 + 10 = 75 + 10 = 85\]
\[76 + 13 + 10 = 89 + 10 = 99\]