(1) Given the set $A=\{x \mid 5 x-a \leqslant 0, a \in \mathbf{N}\}$, if $5 \in A \cap \mathbf{Z}$, then the minimum value of $a$ is $\qquad$ .
(1) 25 Hint: Since $A=\left(-\infty, \frac{a}{5}\right]$, and $5 \in A \cap \mathbf{Z}$, we have $\frac{a}{5} \geqslant 5$, $a \geqslant 25$, therefore the minimum value of $a$ is 25.