Average = a + b + c + d ... / # of values
70 km/h = 50 km/h + x / 2
140 km/h = 50 km/h + x
90 km/h = x
Therefore, the train would have to travel 90 km/h the second half of its journey. The standard average formula I used to solve this ONLY works when you're dealing with equal weights (Each of the two parts of the journal is 50%). If you are faced with a problem in which the parts of the journey are not equal, you will have to use the formula for weighted average, which would be:
Average = x₁y₁ + x₂y₂ + x₃y₃ + ...
In which x = value and y = relative weight (percentage -> decimal)
You can still use this formula to solve your problem.
70 km/h = (50km/h)(0.50)+ x(0.50)
140 km/h = 50 km/h + x
90 km/h = x
