\[\boxed{\mathbf{819\ (819).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[b_{8} = 16;\ \ q = \frac{3}{4}\]
\[b_{8} = b_{7} \cdot q,\ \ \]
\[b_{7} = \frac{b_{8}}{q} = \frac{16 \cdot 4}{3} = \frac{64}{3} = 21\frac{1}{3}.\]