\[\boxed{\text{6.49.}}\]
\[\textbf{а)}\ \frac{1}{2} = \frac{6}{12};\ \]
\[\textbf{б)}\ \frac{1}{4} = \frac{3}{12};\ \]
\[\textbf{в)}\ \frac{3}{4} = \frac{9}{12};\ \]
\[\textbf{г)}\ \frac{2}{6} = \frac{4}{12}\text{.\ \ \ }\]