$a) 10\vdots n\Rightarrow n\in{Ư(10)=\text{{1; 2; 5; 10}}}$
$b)(n+2)\in{Ư(20)=\text{{1; 2; 5; 10; 20}}}$
$\Rightarrow n\in{\text{{-1; 0; 3; 8; 18}}}$
$\text{Vì n}\in{N}$
$\Rightarrow n\in{\text{{0; 3; 8; 18}}}$
$c)12\vdots(n-1)$
$\Rightarrow (n-1)\in{Ư(12)}=\text{{±1; ±2; ±3; ±4; ±6; ±12}}$
$\Rightarrow n\in{\text{{2; 0; 3; -1; 4; -2; 5; -3; 7; -5; 13; -11}}}$
$\text{Vì n}\in{N}$
$\Rightarrow n\in{\text{{2; 0; 3; 4; 5; 7; 13}}}$
$d) (2n+3)\in{Ư(10)=\text{{1; 2; 5; 10}}}$
$\Rightarrow n\in{\text{\{}-1; \frac{-1}{2}; 1; \frac{7}{2}\text{\}}}$
$\text{Vì n}\in{N}$
$\Rightarrow n=1$
