\[\boxed{\mathbf{3.}\mathbf{41}}\]
\[\textbf{а)}\ 83 - 39 = 44\ :4 = 11 \cdot 7 =\]
\[= 77 + 23 = 100\]
\[\textbf{б)}\ 94 - 19 = 75\ :25 = 3 \cdot 15 =\]
\[= 45 + 55 = 100\]
\[\textbf{в)}\ 47 + 25 = 72\ :18 = 4 \cdot 12 =\]
\[= 48 - 19 = 29\]
\[\textbf{г)}\ 71 + 19 = 90\ :15 = 6 \cdot 13 =\]
\[= 78 - 28 = 50\]
\[\textbf{д)}\ 46 + 18 = 64\ :16 = 4 \cdot 25 =\]
\[= 100 - 50 = 50\]