3)
[tex]\it a)\ 3x\leq6,27|_{:3} \Rightarrow x\leq2,09\ \stackrel{x\in\mathbb{N}}{\Longrightarrow}\ x\in\{0,\ 1,\ 2\}\\ \\ \\ b)\ 2(x+1,5)+2<15 \Rightarrow 2x+3+2<15 \Rightarrow 2x+5<15|_{-5} \Rightarrow 2x<10|_{:2} \Rightarrow \\ \\ \Rightarrow x<5\ \stackrel{x\in\mathbb{N}}{\Longrightarrow}\ x\in\{0,\ 1,\ 2,\ 3,\ 4\}[/tex]
4)
[tex]\it \dfrac{a+b+c}{3}=2,8|_{\cdot3} \Rightarrow a+b+c=8,4\ \ \ \ \ \ (1)\\ \\ \\ \dfrac{a+b}{2}=3,5|_{\cdot2} \Rightarrow a+b=7\ \ \ \ \ \ (2)\\ \\ \\ (1),\ (2) \Rightarrow a+b+c-a-b=8,4-7 \Rightarrow c=1,4[/tex]
