\[\boxed{\mathbf{524}.}\]
\[b_{n} = 3 \cdot 2^{- n}\]
\[b_{1} = 3 \cdot \frac{1}{2} = \frac{3}{2};\ \ \]
\[\ b_{2} = 3 \cdot 2^{- 2} = \frac{3}{2^{2}} = \frac{3}{4}.\]
\[q = \frac{3}{4}\ :\frac{3}{2} = \frac{3}{4} \cdot \frac{2}{3} = \frac{1}{2} < 1;\]
\[Прогрессия\ бексонечно\ \]
\[убвающая.\]