\[\boxed{\mathbf{20.}}\]
\[24 + 6 = 30\]
\[24 + 16 = 40\]
\[24 + 26 = 50\]
\[24 + 36 = 60\]
\[32 + 8 = 40\]
\[32 + 18 = 50\]
\[32 + 28 = 60\]
\[32 + 38 = 70\]
\[47 + 3 = 50\]
\[47 + 13 = 60\]
\[47 + 23 = 70\]
\[47 + 33 = 80\]
\[51 + 9 = 60\]
\[51 + 19 = 70\]
\[51 + 29 = 80\]
\[51 + 39 = 90\]