Ta có:
$\frac{x}{2}$ = $\frac{2y}{5}$ = $\frac{3z}{7}$ = $\frac{2x}{4}$ = $\frac{7x}{14}$ = $\frac{4y}{10}$ = $\frac{6z}{14}$
= $\frac{2x+2y-3z}{4+5-7}$ = $\frac{2x+2y-3z}{2}$
= $\frac{7x+4y-6z}{14+10-14}$ = $\frac{7x+4y-6z}{10}$
⇒ A = $\frac{2x+2y-3z}{7x+4y-6z}$ = $\frac{2}{10}$ = $\frac{1}{5}$
Vậy A = $\frac{1}{5}$