17. Вычислите: 24/39 · (1/2 + 7/12) − 8/9 : 10/21.

\[\frac{24}{39}=\frac8{13},\quad \frac12+\frac7{12}=\frac6{12}+\frac7{12}=\frac{13}{12}.\]

\[\frac8{13}\cdot\frac{13}{12}=\frac23,\quad \frac89:\frac{10}{21}=\frac89\cdot\frac{21}{10}=\frac{28}{15}.\]

\[\frac23-\frac{28}{15}=\frac{10-28}{15}=-\frac65.\]

Ответ: \(-\frac65\).