Two Towns

There might be several ways of approaching this problem, here's how I did it:

We find a Common Multiple of 30 and 60: 180.

Then, the time taken to travel from Arrowfront to Boulderfort is 6h (since 180/30 = 6)

The time taken to return from Boulderfort is 3h (since 180/60 = 3)

We calculate the total time and distance for the two trips, which is 9h and 360km repsectively.

Finally, the average driving speed for the two trips was 360/9 = 40km/h.

Given some arbitrary value $x$ as the distance between Arrowfront and Boulderfort, we know that the car took $\displaystyle{{x}\over{30}}+{{x}\over{60}}$ hours to go and come back from Boulderfort. The average time would have to be: $\displaystyle{({{2x}\over{60}}+{{x}\over{60}}) \times 0.5=({{3x}\over{60}}) \times 0.5=({{x}\over{20}})\times 0.5={{x}\over{40}}}$ . Since $x$ represents distance and $40$ represents speed in $km/h$ , the answer must be $\boxed{40}$ .