12. A bag contains 8 white balls and 2 red balls. Each time, one ball is randomly taken out, and then one white ball is put back. What is the probability that all the red balls are exactly taken out by the 4th draw? $\qquad$ .
12.0 .0434$.

The probability of exactly finishing all the red balls on the 4th draw is
$$
\begin{array}{l}
\frac{2}{10} \times\left(\frac{9}{10}\right)^{2} \times \frac{1}{10}+\frac{8}{10} \times \frac{2}{10} \times \frac{9}{10} \times \frac{1}{10}+ \\
\left(\frac{8}{10}\right)^{2} \times \frac{2}{10} \times \frac{1}{10}=0.0434 .
\end{array}
$$