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The displacement of a particle moving in a straight line is 
metres at time 
seconds after leaving a fixed point O. The particle starts from rest and passes through points P, Q and R, at times 
, 
and 
respectively, and returns to O at time 
. The distances OP, OQ and OR are 
, 
and 
respectively. The diagram shows a displacement-time graph which models the motion of the particle from 
to . The graph consists of two curved segments AB and CD and two straight line segments BC and DE. Find the average speed of the particle during its motion.
