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On a trip, a car traveled from town X to town Y and then back from town Y to town X along the same road. What was the car's average speed for the entire round trip?
(1) The one-way distance between X and Y was 120 kilometers.
(2) The car traveled at 60 kilometers per hour from X to Y and at 40 kilometers per hour from Y to X.
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Correct answer
B
Average speed for the whole trip is total distance divided by total time, not the simple average of the two speeds.
(1) alone: the distance is 120, but with no speeds we know neither time nor average speed. Insufficient.
(2) alone: let the one-way distance be d. Going out takes d/60 hours and coming back takes d/40 hours, so total time is d/60 + d/40 = (2d + 3d)/120 = 5d/120 = d/24 hours. Total distance is 2d. Average speed is 2d divided by d/24 = 48 kilometers per hour. The d cancels, so the answer is 48 no matter the distance. sufficient.
The trap is to call the average (60 + 40)/2 = 50. Because the two legs cover equal distance but take unequal time, the slow leg counts for more, pulling the true average down to 48. Statement (2) alone settles it. Answer: (B).
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