\[\boxed{\mathbf{2.557}}\]
\[\textbf{а)}\ 0,51 \cdot \frac{2}{3} = \frac{0,51 \cdot 2}{3} =\]
\[= 0,17 \cdot 2 = 0,34\]
\[\textbf{б)}\ 2,816\ :\frac{4}{7} = \frac{2,816 \cdot 7}{4} =\]
\[= 0,704 \cdot 7 = 4,928\]
\[\textbf{в)}\ \frac{3}{7} \cdot 42,7 = \frac{3 \cdot 42,7}{7} = 3 \cdot 6,1 =\]
\[= 18,3\]
\[\textbf{г)}\ 0,144\ :\frac{12}{13} = \frac{0,144 \cdot 13}{12} =\]
\[= 0,012 \cdot 13 = 0,156\]
\[\textbf{д)}\ 56,25 \cdot \frac{4}{45} = \frac{5625 \cdot 4}{100 \cdot 45} =\]
\[= \frac{125}{25} = 5\]
\[\textbf{е)}\ \frac{21}{23} \cdot 9,2 = \frac{21 \cdot 92}{23 \cdot 10} = \frac{21 \cdot 4}{10} =\]
\[= \frac{84}{10} = 8,4\ \]
\[\textbf{ж)}\ 6,3\ :2\frac{1}{4} = 6,3\ :\frac{9}{4} = 6,3 \cdot \frac{4}{9} =\]
\[= \frac{63 \cdot 4}{10 \cdot 9} = \frac{7 \cdot 4}{10} = \frac{28}{10} = 2,8\]
\[\textbf{з)}\ 6\frac{4}{7} \cdot 5,6 = \frac{46}{7} \cdot 5,6 = \frac{46 \cdot 56}{7 \cdot 10} =\]
\[= \frac{46 \cdot 8}{10} = \frac{368}{10} = 36,8\]