Answer
$t$ varies inversely as $r$
Work Step by Step
It means that if we multiply $r$ by a factor $a$, then $t$ will be multiplied by a factor $\frac{1}{a}$
$t_1=\frac{72}{r_1}$
Now suppose that $r_2=ar_1$
$t_2=\frac{72}{r_2}=\frac{72}{ar_1}=\frac{1}{a}\frac{72}{r_1}=\frac{1}{a}t_1$
$t_2=\frac{1}{a}t_1$
