Boats & Streams · Question 29 · Hard
Boats & Streams question 29
A motorboat leaves a pier going downstream; at the same moment a log starts drifting downstream from the same pier with the current. After going 24 km, the boat turns back at once and meets the log 4 hours after it started, 8 km below the pier. What is the boat's speed in still water?
- 6 km/h
- 10 km/h
- 8 km/h
- 12 km/h
Answer and explanation
Answer: B. 10 km/h
Explanation: Relative to the water (and the log) the boat moves at its still-water speed b both ways, so the trip out and the trip back take equal times: 2 hours each. Downstream speed = 24 ÷ 2 = 12 km/h. The log drifted 8 km in 4 hours, so the stream is 2 km/h and b = 12 − 2 = 10 km/h. Check: back upstream at 8 km/h for 2 hours covers 16 km, reaching the log at 8 km.
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