\[\boxed{\mathbf{3.2}}\]
Пояснение.
Решение.
\[\textbf{а)}\ 0,5\ :2,5 = \frac{0,5}{2,5} = \frac{5}{25} = \frac{1}{5}\]
\[\textbf{б)}\ 6\frac{4}{17}\ :\frac{19}{17} = \frac{106}{17} \cdot \frac{17}{19} = \frac{106}{19}\]
\[\textbf{в)}\ 1,65\ :6\frac{5}{12} = \frac{165}{100}\ :\frac{77}{12} =\]
\[= \frac{33}{50} \cdot \frac{12}{77} = \frac{3 \cdot 6}{25 \cdot 7} = \frac{18}{175}\]