\[\boxed{\mathbf{52}\mathbf{.}}\]
\[28 + 12 = 28 + 10 + 2 =\]
\[= 38 + 2 = 40\]
\[39 + 11 = 39 + 10 + 1 =\]
\[= 49 + 1 = 50\]
\[47 + 13 = 47 + 10 + 3 =\]
\[= 57 + 3 = 60\]
\[56 + 14 = 56 + 10 + 4 =\]
\[= 66 + 4 = 70\]
\[35 + 25 = 35 + 20 + 5 =\]
\[= 55 + 5 = 60\]
\[69 + 21 = 69 + 20 + 1 =\]
\[= 89 + 1 = 90\]
\[33 + 37 = 33 + 30 + 7 =\]
\[= 63 + 7 = 70\]
\[24 + 56 = 24 + 50 + 6 =\]
\[= 74 + 6 = 80\]
\[83 + 17 = 83 + 10 + 7 =\]
\[= 93 + 7 = 100\]
\[75 + 25 = 75 + 20 + 5 =\]
\[= 95 + 5 = 100\]
\[66 + 34 = 66 + 30 + 4 =\]
\[= 96 + 4 = 100\]
\[51 + 49 = 51 + 40 + 9 =\]
\[= 91 + 9 = 100\]