Multiple choice

A car travelled 75% of the distance from town A to town B at an average speed of 50 mph. The car travelled at the average speed of S mph for remaining part of the journey. The average speed for the entire journey was 40 mph. Find the value of S.

  1. 10

  2. 20

  3. 25

  4. 30
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let total distance be D. Time 1 = (0.75D) / 50 = 0.015D. Time 2 = (0.25D) / S. Total time = D / 40 = 0.025D. 0.015D + 0.25D/S = 0.025D. 0.25/S = 0.01. S = 0.25 / 0.01 = 25.

AI explanation

Let the total distance be 100 miles, so the car travels 75 miles at 50 mph and 25 miles at S mph. The time taken for the first part is 75/50 = 1.5 hours and for the second part is 25/S hours. Using the average speed formula, total distance divided by total time equals 40, so 100 / (1.5 + 25/S) = 40. Solving this gives 1.5 + 25/S = 2.5, which means 25/S = 1, so S = 25 mph.

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