Presented solution:

Let the average speed of the train be x. We have

0.8×85.3+0.2×57.3=x

Notice that we need to “weigh” the train speeds - as most of the journey was completed at a certain speed - with a small fraction being completed at a reduced speed (in this case due to “low fuel”).

Solving for x, we x=79.7.

**About this question:**

Initially, we didn’t specify the “time” bit of the question. However, a Reddit user correctly pointed out that if we were to assume distance instead (as could be reasonably assumed with the original formulation of the question), we can’t quite take the weighted average as we did here. See this thread if you’re curious to know why. Fixing this ambiguity made the question look a bit awkward, but we had to do it.

Question: Does multiplying the individual average speeds by time, as done above, not give you the distance instead? I do not get why this is the right answer.