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A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will he take to go 5km in stationery water?

Correct answer: C. 1 hr 15 Min

  • A. 40 Minutes
  • B. 1 Hour
  • C. 1 hr 15 Min
  • D. 1 Hr 30 Min

Explanation

The upstream speed is 2 km ÷ 1 h = 2 km/h, while the downstream speed is 1 km ÷ (10/60 h) = 6 km/h. Their average gives the stationary-water speed: (2 km/h + 6 km/h) ÷ 2 = 4 km/h; therefore, 5 km takes 5 km ÷ 4 km/h = 1.25 h = 1 h 15 min. The likely mistake is choosing 1 hour by using the downstream rate.

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