21. $$x-\frac{x}{8}=\frac{7}{16}$$.

$$\begin{aligned}&x - \frac{x}{8} = \frac{7}{16}\\ &\frac{8x - x}{8} = \frac{7}{16}\\ &\frac{7x}{8} = \frac{7}{16}\\ &7x = \frac{7 \cdot 8}{16}\\ &7x = \frac{7}{2}\\ &x = \frac{7}{2} : 7\\ &x = \frac{7}{2} \cdot \frac{1}{7}\\ &x = \frac{1}{2}\\ &x = 0.5\end{aligned}$$ Ответ: $$\boxed{x = 0.5}$$