what SPEED did it travel on the way back if its average speed for the whole trip was 120 kph?

A train went 300 km from aprovince A to Province B at an average rate of 180 kph. At what speed did it travel on the way back if its average speed for the whole trip was 120 kph?
  a. 90 kph             c. 130 kph   
  b. 120 kph           d. 150 kph

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