Do $x\in\mathbb N$ và $x<131$
$\Rightarrow 0\le x<131$
$\Rightarrow 0\le5q+4<131$
$\Rightarrow -4\le5q<131-4$
$\Rightarrow -4\le5q<127$
$\Rightarrow \dfrac{-4}{5}\le q<\dfrac{127}{5}$
$\Rightarrow q \in \{\dfrac{-4}{5};\dfrac{-3}{5};\dfrac{-2}{5};\dfrac{-1}{5};0;\dfrac{1}{5};\dfrac{2}{5};\dfrac{3}{5};\dfrac{4}{5};...\dfrac{126}{5}\}$
$\Rightarrow x \in \{0;1;2;3;4;5;6;7;8;...130\}$.
$\Rightarrow A=\{0;1;2;3;4;5;...130\}$
