$\frac{15}{60}$ = $\frac{15:15}{60 : 15}$ = $\frac{1}{4}$ ;
$\frac{-7}{5}$ ;
$\frac{16}{15}$ ;
$\frac{28}{-20}$ = $\frac{28:4}{(-20):4}$ = $\frac{-7}{5}$ ;
$\frac{3}{12}$ = $\frac{3:3}{12:3}$ = $\frac{1}{4}$ 
Vậy $\frac{15}{60}$=$\frac{3}{12}$ ; $\frac{-7}{5}$=$\frac{28}{-20}$