\[\boxed{\mathbf{3.57}}\]
\[\textbf{а)}\ 3,06\ :0,9 = 4,08\ :1,2\]
\[306\ :90 = 408\ :120\]
\[3,4 = 3,4 \rightarrow верно.\]
\[\textbf{б)}\ 0,0056\ :0,14 = 0,136\ :0,34\]
\[56\ :1400 = 136\ :340\]
\[0,04 = 0,4 \rightarrow неверно.\]