Multiply by the denominator, isolate r terms, then divide by the coefficient of r.
[tex]t=\dfrac{120r}{r+120}\qquad\text{given}\\\\t(r+120)=120r\qquad\text{multiply by $(r+120)$}\\\\120t=120r-tr\qquad\text{subtract $tr$}\\\\\dfrac{120t}{120-t}=r\qquad\text{divide by the coefficient of $r$}\\\\\boxed{r=\dfrac{120t}{120-t}}[/tex]
