Suppose $c$ and $d$ are inversely proportional. If $c=9$ when $d=8$, find the value of $d$ when $c=6$.
I know that inverse proportionality means that the product of $c$ and $d$ is constant, so I can write an equation: $cd=k$, where $k$ is some constant. I can use the given information to find $k$: if $c=9$ and $d=8$, then $k=9\times 8=72$. Now I can use this equation to find $d$ when $c=6$: I just plug in $c=6$ and solve for $d$: $6d=72$, so $d=72/6=12$. 