Chapter 1, Equations and Graphs - Section 1.10 - Modeling Variation - 1.10 Exercises - Page 164: 26

$t=50 \frac{xy}{r}$

Since $t$ is directly (jointly) proportional to the product of $x$ and $y$ and inversely proportional to $r$, we have: $t=k \frac{xy}{r}$ We know: $t=25$ and $x=2$ and $y=3$ and $r=12$, so we have: $25=k \frac{2*3}{12}$ $ k=25*12/6=25*2=50$ So the equation is: $t=50 \frac{xy}{r}$