Find $x$ from $xy = 12$, $yz = 20$, and $zx = 15$

Multiply
$xy \cdot yz \cdot zx = 12 \cdot 20 \cdot 15$

$x^2 y^2 z^2 = 3600$

$(xyz)^2 = 3600$

$xyz = 60$
 

Divide
$\dfrac{xyz}{yz} = \dfrac{60}{20}$

$x = 3$   ←   answer