\[\boxed{\mathbf{18.}}\]
\[38 + 12 = 38 + 10 + 2 =\]
\[= 48 + 2 = 50\]
\[27 + 13 = 27 + 10 + 3 =\]
\[= 37 + 3 = 40\]
\[46 + 14 = 46 + 10 + 4 =\]
\[= 56 + 4 = 60\]
\[24 + 26 = 24 + 20 + 6 =\]
\[= 44 + 6 = 50\]
\[49 + 31 = 49 + 30 + 1 =\]
\[= 79 + 1 = 80\]
\[53 + 27 = 53 + 20 + 7 =\]
\[= 73 + 7 = 80\]
\[62 + 28 = 62 + 20 + 8 =\]
\[= 82 + 8 = 90\]
\[75 + 15 = 75 + 10 + 5 =\]
\[= 85 + 5 = 90\]
\[26 + 54 = 26 + 50 + 4 =\]
\[= 76 + 4 = 80\]