[tex]r-\text{the weight of the rocks}\\\\\dfrac{3}{7}r-\text{the weight of the truck}\\\\13.5t-\text{the weight of the truck}\\\\\text{The equation}\\\\\dfrac{3}{7}r=13.5\qquad\text{multiply both sides by}\ \dfrac{7}{3}\\\\\boxed{r=31.5\ (t)}\\\\Answer:\ The\ weight\ of\ rocks\ is\ 31.5\ tons.[/tex]
