The formula for simple interest is I=PrtI = PrtI=Prt. Solve for rrr.
r=IPtr = \frac{I}{Pt}r=PtI
r=I−Ptr = I - Ptr=I−Pt
r=PtIr = \frac{Pt}{I}r=IPt
r=IP+tr = \frac{I}{P+t}r=P+tI