A boats man can row in still water at speed of 7 km/hr. It takes 6 hours more to travel the same distance in upstream than in downstream if the speed of the river is 3 km/hr. what is the distance between the two destinations?
Correct answer: B. 40 km
- A. 20 km
- B. 40 km
- C. 30 km
- D. 10 km
Explanation
The upstream speed is 7 - 3 = 4 kmph and the downstream speed is 7 + 3 = 10 kmph. If the distance is D km, D/4 - D/10 = 6 hours, so 3D/20 = 6 and D = 40 km. The likely choice is c, 30 km, from subtracting speeds or times without applying the distance to both journeys.
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About Time, Speed and Distance
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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