Question

$x$ varies inversely as $y$, when $x=15$ then $y=10$, if $x=20$ then $y=?$

Answer 1

It is given that x varies inversely as y i.e $x\propto$ $\frac{1}{y}$
∴ $x=\frac{k}{y}$, where $k$ is constant of variation
⇒ $xy$ $=k$
When $x=15,y=10$
∴ $k=15\times 10=150$
So, the equation of variation is $xy=k=150$
When, $x=20$
⇒ $20y=150$
⇒ $y=\frac{150}{20}=7.5$