#### Answer

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

#### Work Step by Step

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