Question

If $x$ varies inversely as $y$ and $x=20$ when $y=600$, find $y$ when $x=400$

Answer 1

If $x$ varies inversely with $y$
$xy=k$ (constant) ...... (i)
If $x=20$ and $y=600$
$Xy=20\times 600=12000$
$k=12000$
When $x=400$ then from equation $\left(i\right)$
$y\times 400=k$
$y\times 400=12000$
$y=\frac{12000}{400}=30$