Boats & Streams · Question 23 · Medium
Boats & Streams question 23
A motorboat with a still-water speed of 15 km/h goes 30 km downstream and comes back in a total of 4 1/2 hours. What is the speed of the stream?
- 4 km/h
- 3 km/h
- 5 km/h
- 6 km/h
Answer and explanation
Answer: C. 5 km/h
Explanation: 30/(15 + s) + 30/(15 − s) = 4.5 gives 900/(225 − s²) = 4.5, so 225 − s² = 200 and s = 5 km/h. Check: 30/20 + 30/10 = 1.5 + 3 = 4.5 hours.
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