
$800$∶$680$$=\left ( {800\div 10} \right )$∶$\left ( {680\div 10} \right )$
$=80$∶$68$$=\left ( {80\div 4} \right )$∶$\left ( {68\div 4} \right )$
$=20$∶$17$

$800$∶$680$$=\left ( {800\div 10} \right )$∶$\left ( {680\div 10} \right )$
$=80$∶$68$$=\left ( {80\div 4} \right )$∶$\left ( {68\div 4} \right )$
$=20$∶$17$