\[5\frac{1}{3}\ :\left( \frac{8}{9}\ :\frac{1}{3} \right) = \frac{16}{3}\ :\frac{8 \cdot 3}{9 \cdot 1} =\]
\[= \frac{16}{3}\ \ :\frac{8}{3} = \frac{16 \cdot 3}{3 \cdot 8} = 2\]