The distance between stations A and B is 778 kilometres. A train travels from station A to station B at a uniform speed of 84 km/h and then returns to station A at a uniform speed of 56 km/h. What is the train's average speed (in km/h) throughout the journey?

67.2

Average Speed = \(\frac{Total\; Distance }{Total\; Time }\)

Distance = Speed × Time

Total distance = 778 + 778 = 1556 km

Time taken while going from A to B = \(\frac{778 }{84 }\) = 9.26 hours

Time taken while returning back to  A = \(\frac{778 }{56 }\) = 13.89 hours

Average speed =   \(\frac{1556  }{ 9.26 + 13.89 }\)

= \(\frac{1556  }{ 23.15 }\) 

= 67.2 km/h