A person starts travelling from a point P towards R passes through Q which is between P and R. The time taken to travel from P to Q is half that of the time taken from Q to R. However, after passing through any of the given points, he travels at a speed which is two-thirds that of the previous journey. Find the time taken, in seconds, to travel from P to R and back to P, if he always passes through Q in the journey and takes 15 minutes to go from P to Q initially.