The ratio of weights of copper and zinc in brass is $13:7$. Find the weight of zinc in a brass utensil weighing $700gm$.

Let proportionality constant be 'x'

then, weight of copper $=13x$

and weight of zinc $=7x$

As per the question

Weight of brass $=700gm$

Weight of copper in brass + Weight of zinc in brass $=700gm$

$\Rightarrow 13x+7x=700$

$\Rightarrow 20x=700$

$\Rightarrow x=35$

Hence, Weight of copper in brass $=13\times 35=455gm$

Weight of zinc in brass $=7\times 35=245gm$