3. In the Cartesian coordinate system $x O y$, the moving point $M(x, 0)$ on the $x$-axis has distances $M P$ and $M Q$ to the fixed points $P(5,5)$ and $Q(2,1)$, respectively. When $M P + M Q$ takes the minimum value, the $x$-coordinate of point $M$ is $x=$ $\qquad$ . (2001, TI Cup National Junior High School Mathematics Competition)
(Answer: $\frac{5}{2}$ )