According to Kepler's third law of planetary motion, what is the relationship between 'r' (radius of orbit) and 'T' (period of revolution)?

According to Kepler's third law of planetary motion, the square of the period of revolution (T) of a planet is directly proportional to the cube of the radius of its orbit (r). 

This can be expressed as: 

$$\begin{gathered} T^2\propto r^3 \\ \Rightarrow \frac{r^3}{T^2}\text{ is constant} \end{gathered}$$.