Multiple choice

J drives to the store at 20 mph and returns by the same route at 30 mph. Excluding the time spent at the store, what was J`s average speed (in mph)?

  1. 23.5

  2. 24

  3. 24.5

  4. 25

  5. 26
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Average speed is total distance divided by total time. If the distance to the store is d, the total distance is 2d. The time taken is d/20 + d/30 = (3d + 2d)/60 = 5d/60 = d/12. Average speed = 2d / (d/12) = 24 mph.

AI explanation

Use the harmonic mean formula for equal distances: average speed equals two times the product of the speeds divided by their sum. Multiplying 20 and 30 gives 600, and multiplying that by 2 gives 1200. Adding the two speeds gives 50, and dividing 1200 by 50 gives an average speed of 24 mph.

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