Giải thích các bước giải:

28.Ta có:

$\dfrac{2x}3=\dfrac{3y}4=\dfrac{4z}5$

$\to \dfrac{2x}3:12=\dfrac{3y}4:12=\dfrac{4z}5:12$

$\to \dfrac{x}{18}=\dfrac{y}{16}=\dfrac{z}{15}=\dfrac{x+y+z}{18+16+15}=\dfrac{49}{49}=1$

$\to x=18, y=16, z=15$

29.Ta có:

$\dfrac6{11}x=\dfrac92y=\dfrac{18}5z$

$\to \dfrac6{11}x:18=\dfrac92y:18=\dfrac{18}5z:18$

$\to \dfrac{x}{33}=\dfrac{y}4=\dfrac{z}5=\dfrac{-x+y+z}{-33+4+5}=\dfrac{-120}{-24}=5$

$\to x= 165, y=20, z=25$

30.Ta có:

$\dfrac6{11}x=\dfrac92y=\dfrac{18}5z$

$\to \dfrac6{11}x:18=\dfrac92y:18=\dfrac{18}5z:18$

$\to \dfrac{x}{33}=\dfrac{y}4=\dfrac{z}5=\dfrac{-x+y+z}{-33+4+5}=\dfrac{-196}{-24}=\dfrac{49}6$

$\to x=33\cdot \dfrac{49}6=\dfrac{539}2$

     $y=4\cdot \dfrac{49}6=\dfrac{98}3$

     $z=5\cdot \dfrac{49}6=\dfrac{245}6$