a)
$\frac{3}{7}$ = $\frac{3 x 5}{7 x 5}$ = $\frac{15}{35}$
$\frac{2}{5}$ = $\frac{2 x 7}{5 x 7}$ = $\frac{14}{35}$
$\frac{15}{35}$ > $\frac{14}{35}$ so $\frac{3}{7}$ > $\frac{2}{5}$
b)
$\frac{5}{9}$ = $\frac{5 x 8}{9 x 8}$ = $\frac{40}{72}$
$\frac{5}{8}$ = $\frac{5 x 9}{8 x 9}$ = $\frac{45}{72}$
$\frac{45}{72}$ > $\frac{40}{72}$ so $\frac{5}{8}$ > $\frac{5}{9}$
c) $\frac{8}{7}$ = $\frac{8 x 8}{7 x 8}$ = $\frac{64}{56}$
$\frac{7}{8}$ = $\frac{7 x 7}{8 x 7}$ = $\frac{49}{56}$
$\frac{49}{56}$ < $\frac{64}{56}$ so $\frac{7}{8}$ < $\frac{8}{7}$