\[\boxed{\mathbf{6.75}}\]
\[\textbf{а)}\ x\ :3,5 = 1,2\ :0,4\]
\[x\ :3,5 = 3\]
\[x = 3 \cdot 3,5\]
\[x = 10,5.\]
\[\textbf{б)}\ 2,5\ :6,8 = 1,5\ :y\]
\[2,5y = 6,8 \cdot 1,5\]
\[y = \frac{6,8 \cdot 1,5}{2,5} = \frac{68 \cdot 3}{5} = \frac{204}{5}\]
\[y = 40\frac{4}{5}\]
\[y = 40,8.\]