б) \(\frac{4}{5}-\left(\frac{9}{10}-z\right)=\frac{1}{5}\);

Решим уравнение:

$$ \frac{4}{5}-\left(\frac{9}{10}-z\right)=\frac{1}{5} $$

$$ \frac{4}{5} - \frac{9}{10} + z = \frac{1}{5} $$

$$ z = \frac{1}{5} - \frac{4}{5} + \frac{9}{10} $$

$$ z = \frac{2}{10} - \frac{8}{10} + \frac{9}{10} $$

$$ z = \frac{2 - 8 + 9}{10} $$

$$ z = \frac{3}{10} $$

Ответ: $$z = \frac{3}{10}$$