Răspuns :
[tex]\it 2x+7<20\Big|_{-7} \Rightarrow 2x<13\ \stackrel{x\in\mathbb{N}}{\Longrightarrow}\ A=\{0,\ \ 1,\ \ 2,\ \ 3,\ \ 4,\ \ 5,\ \ 6\}[/tex]
[tex]\it 2x+7<20\Big|_{-7} \Rightarrow 2x<13\ \stackrel{x\in\mathbb{N}}{\Longrightarrow}\ A=\{0,\ \ 1,\ \ 2,\ \ 3,\ \ 4,\ \ 5,\ \ 6\}[/tex]