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\(\dfrac{1}{5} :\) [ ] \(= \dfrac{3}{5}\) ;

\(\dfrac{1}{7} : \) [ ] \(= \dfrac{4}{7}\) ;

\(4 :\) [ ] \(=8\) ;

\(5 :\) [ ] \(= 15\) ;

[ ] \(: 3 = \dfrac{1}{12}\) ;

[ ] \( : 5 = \dfrac{1}{30}\) .