When a question describes a trip in which someone travels at two different speeds and then asks for the average speed of the
entire trip, the answer is NEVER the average of the two speeds. Average speed is always total distance divided by total time.
Before the piston broke, the car took one hour to travel 60 kilometers. Afterwards, it traveled another 60 kilometers at a
speed of 30 kilometers per hour, so it took 2 hours to travel the second 60 kilometers.
Total distance: 60 km + 60 km = 120 km
Total time 1hr + 2 hr = 3 hr
Average speed: choice (B).
Choice (C) is a trap—it is the average of the two speeds.