A cyclist rides 18 km at 9 km/h and then 24 km at 16 km/h. What is her average speed for the whole journey?

Answer and explanation

Correct answer: C. 12 km/h

  1. Step 1: Time for the first part: 18 ÷ 9 = 2 hours. Time for the second: 24 ÷ 16 = 1.5 hours. Step 2. Total distance 42 km in 3.5 hours. Step 3. 42 ÷ 3.5 = 12 km/h. Answer: 12 km/h. Why not the others: 12.5 km/h: This is the mean of the two speeds, which is wrong because she spends different times at each speed. 14 km/h: This divides 42 km by 3 hours, forgetting that the second part takes an extra half hour. 21 km/h: This divides the 42 km by 2, the number of stages, instead of by the total time.

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