:D nhanh nèeeeee

a) $\dfrac{4}{9}$  : $\dfrac{2}{3}$ = $\dfrac{4}{9}$ x $\dfrac{3}{2}$ = $\dfrac{4 x 3}{9x 2}$ = $\dfrac{12}{18}$ = $\dfrac{2}{3}$

b) $\dfrac{32}{8}$ : $\dfrac{8}{21}$ = $\dfrac{32}{8}$ x $\dfrac{21}{8}$ = $\dfrac{32 x 21}{8 x 8}$ = $\dfrac{672}{64}$ = $\dfrac{21}{2}$

c ) $\dfrac{16}{21}$ : $\dfrac{32}{28}$ =  $\dfrac{16}{21}$ x $\dfrac{28}{32}$ = $\dfrac{16 x 28}{21 x 32}$ = $\dfrac{448}{672}$ = $\dfrac{3}{8}$

d ) $\dfrac{18}{64}$ : $\dfrac{9}{8}$ = $\dfrac{18}{4}$ x $\dfrac{8}{9}$ = $\dfrac{18 x 8}{4 x 9}$ = $\dfrac{144}{36}$ = $\dfrac{4}{1}$ = 4