【Question 3】
The store has a safe, the password is $3854 \square 942$, and the digit in the fifth position from left to right is forgotten, but it is remembered that the password is the product of $5678 \times 6789$. So, what should be filled in $\square$?
【Analysis and Solution】Number Theory, Congruence.
$$
\begin{array}{l}
\overline{3854 \square 942} \equiv 3+8+5+4+\square+9+4+2=35+\square \equiv 3+5+\square=8+\square(\bmod 9) ; \\
5678 \times 6789 \equiv(5+6+7+8) \times(6+7+8+9)=26 \times 30 \equiv(2+6) \times(3+0)=8 \times 3=24 \equiv 2+4=6(\bmod 9) ;
\end{array}
$$

Since $5678 \times 6789=\overline{3854 \square 942}$; therefore $8+\square \equiv 6(\bmod 9)$; so $\square=9+6-8=7$.