[tex]\it \{x,\ y,\ z\}\ i.\ p.\ \{2,\ 4,\ 8\}\ \Rightarrow\ 2x=4t=8z\bigg|_{:2} \Rightarrow x=2y=4z \Rightarrow \begin{cases} \it y=\dfrac{x}{2}\\ \\ \it z=\dfrac{x}{4}\end{cases}\ (*)\\ \\ \\ x+y+z=28\ \stackrel{(*)}{\Longrightarrow}x+\dfrac{x}{2}+\dfrac{x}{4}=28\bigg|_{\cdot4} \Rightarrow 4x+2x+x=4\cdot28 \Rightarrow \\ \\ \\ 7x=4\cdot28\bigg|_{:7} \Rightarrow x=4\cdot4 \Rightarrow x=16\\ \\ \\y=\dfrac{x}{2}=\dfrac{16}{2}=8;\ \ \ z=\dfrac{x}{4}=\dfrac{16}{4}=4[/tex]
