\[\boxed{\mathbf{72}\mathbf{.}}\]
\[100 - 15 = 100 - 10 - 5 =\]
\[= 90 - 5 = 85\]
\[100 - 25 = 100 - 20 - 5 =\]
\[= 80 - 5 = 75\]
\[100 - 35 = 100 - 30 - 5 =\]
\[= 70 - 5 = 65\]
\[100 - 13 = 100 - 10 - 3 =\]
\[= 90 - 3 = 87\]
\[100 - 42 = 100 - 40 - 2 =\]
\[= 60 - 2 = 58\]
\[100 - 51 = 100 - 50 - 1 =\]
\[= 50 - 1 = 49\]
\[100 - 37 = 100 - 30 - 7 =\]
\[= 70 - 7 = 63\]
\[100 - 64 = 100 - 60 - 4 =\]
\[= 40 - 4 = 36\]
\[100 - 72 = 100 - 70 - 2 =\]
\[= 30 - 2 = 28\]