\[\boxed{\mathbf{3.127}}\]
\[\textbf{а)}\ \frac{0,21 \cdot 1,25}{13,6 - 11,1} = \frac{0,21 \cdot 1,25}{2,5} =\]
\[= 0,21 \cdot 0,5 = 0,105\]
\[\textbf{б)}\ \frac{2,781}{2,06} + \frac{7,825}{3,13} =\]
\[= \frac{2781}{2060} + \frac{7825}{3130} = 1,35 + 2,5 =\]
\[= 3,85\]