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A car travels first 160 km at 64 km/hr and the next 160 km at 80 km/hr. What is the average speed for the first 320 km of the tour?

Correct answer: C. 71.11 km/hr

  • A. 70.24 km/hr
  • B. 74. 24 km/hr
  • C. 71.11 km/hr
  • D. 72.21 km/hr

Explanation

The first 160 km takes 160 km / 64 km/h = 2.5 hours, and the next 160 km takes 160 km / 80 km/h = 2 hours. Total distance is 320 km and total time is 4.5 hours, so average speed = 320 km / 4.5 hours = 71.11 km/h. The likely wrong choice is 72.21 km/h, which comes from averaging the two speeds without giving equal weight to their travel times.

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About Time, Speed and Distance

Problems relate distance, speed and time through the formula distance = speed × time, including unit conversion, average speed, relative speed and motion in opposite or the same direction. Questions may involve trains, boats and streams, where the effective speed changes, so average speed must not be confused with the simple average of two speeds.

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