CMA Foundation · Fundamentals of Business Mathematics and Statistics · Time and Distance
A boat's speed in still water is 15 km/h and the speed of the stream is 5 km/h. What is the boat's average speed for a round trip, going a certain distance upstream and returning to the starting point downstream?
Average speed over equal distances is the harmonic mean of the upstream and downstream speeds, 10 and 20 km/h, which gives 13.33 km/h.
- A10 km/h
- B12.5 km/h
- C11.25 km/hCorrect
- D15 km/h
Explanation
Upstream speed = 15 - 5 = 10 km/h and downstream speed = 15 + 5 = 20 km/h. For equal distances, average speed = 2 x 10 x 20 / (10 + 20) = 400/30 = 13.33 km/h. Check with 20 km each way: time = 2 + 1 = 3 h, distance 40 km, so 13.33 km/h. The listed option 11.25 is therefore wrong, so the correct value must be recomputed as 13.33.
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