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Boats & Streams, Pipes & Cisterns · Question 34 of 85

What is the swimmer's average speed for the whole swim, there and back?

Question 34

MediumOriginal question

A swimmer swims at 4 km/h in still water. In a river flowing at 2 km/h, she swims 3 km downstream and then swims back to where she started.

What is the swimmer's average speed for the whole swim, there and back?

Options for question 34

Show the answer and explanation
CorrectNot quiteAnswer C

Correct answer: C

Explanation

  1. 6 km in 0.5+1.5=20.5 + 1.5 = 2 hours.

  2. Average speed =62=3= \frac{6}{2} = 3 km/h.

Worth noting

4 km/h averages 6 and 2 as if she spent equal times at each; the slow leg takes three times as long.

Concept
The average speed of a round trip
Typical time
120 s
Your time
—
The rule this testsJourneys on a river
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Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

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Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

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