A man can row three-quarters of a kilometer against the stream in 11 ¼ minutes and return in 7 ½ minutes. The speed of the man in still water is________?
Correct answer: D. 5 km/hr
- A. 2km/hr
- B. 3km/hr
- C. 4 km/hr
- D. 5 km/hr
Explanation
The upstream speed is 0.75 km ÷ 0.1875 hour = 4 km/hr, and the downstream speed is 0.75 km ÷ 0.125 hour = 6 km/hr. Still-water speed is the average, (4 + 6) ÷ 2 = 5 km/hr. A student may choose 4 km/hr by giving only the upstream speed instead of removing the current's effect.
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Problems relate distance, speed and time through the formula distance = speed × time, including unit conversion, average speed, relative speed and motion in opposite or the same direction. Questions may involve trains, boats and streams, where the effective speed changes, so average speed must not be confused with the simple average of two speeds.
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