Answer
$C=16lwh$
Work Step by Step
$C$ is jointly proportional to $l, w,$ and $h$ and inversely proportional to $r$ so the equation that relates the variables is:
$C=k(lwh)$ where k = constant of proportionality.
If l = w = h = 2, then C = 128. Substitute these values into the equation above to have:
$C=k(lwh)
\\128 = k(2\cdot 2\cdot 2)
\\128=8k
\\\dfrac{128}{8}=k
\\16=k$
The constant of proportionality is $16$. Thus, the equation that relates C, l, w, and h is:
$C=16(lwh)
\\C=16lwh$
