Ha $9^{-1 \frac{1}{6}}=0.0770401$, what is $9^{-\frac{2}{3}}$?
$$
9^{-\frac{2}{3}}=9^{-1 \frac{1}{6}} \cdot 9^{\frac{1}{2}}=9^{-1 \frac{1}{6}} \cdot 3=0.2311203
$$

Kertész Gusztáv, Pécs.

The problem was also solved by: Déri Zs., Deutsch I., Haar A., Harsányi Z., Kürti I., Raab R., Ragány B., Simonyi Gy.