$\frac{8}{17}$ и $\frac{11}{21}$
8 * 21 = 168;
17 * 11 = 187;
168 < 187, то
$\frac{8}{17} < \frac{11}{21}$.

0,6 и $\frac{4}{7}$
$0,6 = \frac{6}{10} = \frac{3}{5} = \frac{21}{35}$;
$\frac{4}{7} = \frac{20}{35}$;
$\frac{21}{35} > \frac{20}{35}$, то
$0,6 > \frac{4}{7}$.

$\frac{6}{25}$ и 0,219
$\frac{6}{25} = \frac{24}{100} = 0,24$;
0,24 > 0,219, то
$\frac{6}{25} > 0,219$.