As the train’s engineer, erin guided the train on a 64 mi run, which took 1.7 h. she then stopped on a side rail to let a second train pass. she then continued another 80 mi, which took 1.9 h.

what was the train’s average speed for the entire run?

Let's calculate how many miles the train has passed in total:
64+80=144 miles

let's also calculate the complete time: (we can ignore the waiting time here, since they also didn't move during this time:

1.7h+1.9h=3.6 hours.

And now we only need to divide the distance per time:

So the correct answer is 40 miles per hour!