$+) \quad 12x = 5y$
$\Leftrightarrow \dfrac{x}{5} = \dfrac{y}{12}$
$+) \quad 7y = 3z$
$\Leftrightarrow \dfrac{y}{3} = \dfrac{z}{7}$
$\Leftrightarrow \dfrac{y}{12} = \dfrac{z}{28}$
$\Rightarrow \dfrac{x}{5} = \dfrac{y}{12} = \dfrac{z}{28}$
$\Rightarrow \dfrac{3x}{15} = \dfrac{2y}{24} = \dfrac{z}{28}$
Áp dụng tính chất dãy tỉ số bằng nhau, ta được:
$\dfrac{3x}{15} = \dfrac{2y}{24} = \dfrac{z}{28} = \dfrac{3x + 2y - z}{15 + 24 - 28} = \dfrac{121}{11} = 11$
$+) \quad \dfrac{x}{5} = 11 \Leftrightarrow x = 5.11 = 55$
$+) \quad \dfrac{y}{12} = 11 \Leftrightarrow y = 12.11 = 132$
$+) \quad \dfrac{z}{28} = 11 \Leftrightarrow z = 28.11 = 308$
