11. The product of the two ten-digit numbers 1111111111 and 9999999999 has how many odd digits?
11. 【Solution】1111111111×9999999999
$$
\begin{array}{l}
=1111111111 \times(10000000000-1) \\
=11111111110000000000-1111111111 \\
=11111111108888888889
\end{array}
$$

Thus, there are 10 digits that are odd.