\[\boxed{\mathbf{2.441}}\]
\[\textbf{а)}\ \frac{1}{3} \cdot \frac{3}{7} = \frac{1}{7}\]
\[\textbf{б)}\ \frac{1}{21} \cdot 4\frac{1}{5} = \frac{1}{21} \cdot \frac{21}{5} = \frac{1}{5}\]
\[\textbf{в)}\ 1\frac{1}{3} \cdot \frac{3}{4} = \frac{4}{3} \cdot \frac{3}{4} = 1\]
\[\textbf{г)}\ 3\frac{2}{3} \cdot \frac{6}{11} = \frac{11}{3} \cdot \frac{6}{11} = 2\]
\[\textbf{д)}\ \left( \frac{1}{5} + \frac{1}{20} \right) \cdot \frac{4}{5} = \frac{1}{5} \cdot \frac{4}{5} + \frac{1}{20} \cdot \frac{4}{5} =\]
\[= \frac{4}{25} + \frac{1}{25} = \frac{5}{25} = \frac{1}{5}\]
\[\textbf{е)}\ \left( \frac{1}{3} - \frac{1}{4} \right) \cdot 12 =\]
\[= \frac{1}{3} \cdot 12 - \frac{1}{4} \cdot 12 = 4 - 3 = 1\]