Multiple choice

In a stream running at 2 km/hr, a motor boat goes 10 km upstream and back again to the starting point in 55 min. Find the speed of the motorboat in still water.

  1. $24 km/hr$
  2. $20 km/hr$
  3. $22 km/hr$
  4. $21 km/hr$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the boat's speed in still water be x km/hr. Upstream speed is (x-2) km/hr and downstream speed is (x+2) km/hr. Time upstream + time downstream = 10/(x-2) + 10/(x+2) = 55/60 hours. Solving: 20x/(x²-4) = 11/12, which gives 11x² - 240x - 44 = 0. The valid solution is x = 22 km/hr.

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