Решите уравнение: (20-(3a-7))/10=9/10.

\[\frac{20 - (3a - 7)}{10} = \frac{9}{10}\ \ \ \ | \cdot 10\]

\[20 - (3a - 7) = 9\]

\[3a - 7 = 20 - 9\]

\[3a - 7 = 11\]

\[3a = 11 + 7\]

\[3a = 18\]

\[a = 18\ :3\]

\[a = 6.\]

\[Ответ:a = 6.\]