Question

If $x$ varies directly as $y$ and if $x=4$ when $y=20$, find $y$ when $x=12$

Answer 1

We know that $x\propto y$ and $x4$ when $y=20$. Thus $x=ky$ for some constant $k$. Putting $x=4$ and $y=20$ in this, we get $4=20k$ or $k=1/5$. Hence
Now take $x=12$ in this. We obtain $y=12\times 5=60$.