[tex] \frac{x}{6} = \frac{y}{7} = \frac{z}{8} = k[/tex]
[tex]x = 6k[/tex]
[tex]y = 7k[/tex]
[tex]z = 8k[/tex]
[tex]x + y - z = 20[/tex]
[tex]6k + 7 k- 8k = 20[/tex]
[tex]5k = 20[/tex]
[tex]k = 4[/tex]
[tex]x = 6 \times 4 = 24[/tex]
[tex]y = 7 \times 4 = 28[/tex]
[tex]z = 8 \times 4 = 32[/tex]