Official Solution:
If a car traveled at a constant speed of 75 km/h during the first two hours of its four-hour trip, what was the average speed of the car for the entire journey?
(1) During the last two hours of the trip, the car covered 10 km more than it did during the first two hours.
In the first two hours, the car traveled \(75*2 = 150\) km. In the last two hours, it traveled \(150 + 10 = 160\) km. The average speed is given by \(\frac{total \ distance}{total \ time}=\frac{150 + 160}{4} = 77.5\) km/h. Sufficient.
(2) The average speed of the car during the last two hours of the trip was 80 km/h.
In the first two hours, the car traveled \(75*2 = 150\) km. In the last two hours, it traveled \(80*2 = 160\) km. The average speed is \(\frac{total \ distance}{total \ time}=\frac{150 + 160}{4} = 77.5\) km/h. Sufficient.
Answer: D