Lời giải:
$30\vdots x; 45\vdots x; 120\vdots x$
$\Rightarrow x=ƯC(30,45,120)$
$\Rightarrow ƯCLN(30,45,120)\vdots x$
$\Rightarrow 15\vdots x$
$\Rightarrow x\in \left\{1; 3; 5; 15\right\}$
Mà $3\leq x\leq 10$ nên $x=3$ hoặc $x=5$
$\Rightarrow A=\left\{3; 5\right\}$