Official Solution: A car covered half of the total distance at a speed of 60 kilometers per hour, and then increased its speed for the second half of the trip. If the total time taken for the trip was 3 hours, what was the average speed of the car for the second half of the journey? (1) The car took 2 hours to cover the first half of the distance.
For the first
half of the distance, the car traveled at a speed of 60 kilometers per hour for 2 hours, covering \(60*2=120\) kilometers. Thus, the second
half of the journey is also 120 kilometers. Since the car spent \(3-2=1\) hour on the second half of the journey, the average speed for the second half was \(speed=\frac{distance}{time}=\frac{120}{1}=120\) kilometers per hour. Sufficient.
(2) The total distance covered was 240 kilometers.
Half of this distance is 120 kilometers. To cover that distance at a speed of 60 kilometers per hour, the car needed \(time=\frac{distance}{speed}=\frac{120}{60}=2\) hours. Therefore, it spent \(3-2=1\) hour covering the second half of the distance. The average speed for the second half of the distance was \(speed=\frac{distance}{time}=\frac{120}{1}=120\) kilometers per hour. Sufficient.
Answer: D
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