\[\boxed{\mathbf{624\ (624)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ не\ нарушится:\]
\[x\ :y = a\ :b\]
\[\frac{x}{4}\ :\frac{y}{4} = \frac{a}{b}\]
\[\frac{x}{4} \cdot \frac{4}{y} = \frac{a}{b}\]
\[\frac{x}{y} = \frac{a}{b}.\]
\[2)\ нарушится:\]
\[x\ :y = a\ :b\]
\[x\ :10y = 10a\ :b\]
\[\frac{x}{10y} = \frac{10a}{b}.\]
\[3)\ не\ нарушится:\]
\[x\ :y = a\ :b\]
\[6x\ :y = 6a\ :b\]
\[\frac{6x}{y} = \frac{6a}{b}.\]