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Call the unknown speed of the current c and the speed of the boat in still water b. When travelling upstream, the net speed of the boat will be b - c and when travelling downstream the net speed of the boat will be b + c. Since b = 4c, the speed upstream will be 3c. The distance upstream is one-half the total travelling distance of 150 km or 75 kilometers. Distance travelled = speed X time at speed, so that upstream time = 75/3c, downstream time = 75/5c, and the sum of these is the total time stated to be 8 hours. Thus 75/3c + 75/5c = 8. Multiplying both sides by 15c yields 75(5 +3) = 120c, or c = (8 X 75)/120 = 5 kilometers per hour.

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Q: In still water a boat can travel four times as fast as the current in the river A trip up the river and back which totaled 150 km took 8h Find the speed of the current?
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