Răspuns :

Folosim proportii derivate
x/y=3/7 => x/3=y/7  (1)      
y/z=7/12 => y/7=z/12  (2)  
  Din relatiile (1) si (2) =>x/3=z/12 => x/z=3/12
  Valoarea acestui raport este 3:12=1:4= 0,25
[tex] \frac{x}{y} = \frac{3}{7}=>x= \frac{3}{7}*y \\ \frac{y}{z}= \frac{7}{12}=>z= \frac{y}{ \frac{7}{12}}=y* \frac{12}{7} \\ \frac{x}{z}= \frac{y* \frac{3}{7}}{y* \frac{12}{7}}= \frac{\frac{3}{7}}{\frac{12}{7}}= \frac{3}{7}* \frac{7}{12}= \frac{3}{12} [/tex]