A person goes from P to Q at a speed of 20 km/h. Then he goes from Q to R at a speed of q km/h. Finally the person goes from

R to S at a speed of r km/h. The distances from P to Q, Q to R and R to S are equal. If the average speed from P to R is  km/h,

and the average speed from Q to S is  km/h, then what is the value of r?