A motorboat, whose speed in 15 km/hr in still water goes 27 km downstream and comes back in a total of 3 hours 45 minutes. The speed of the stream (in km/hr) is: A. 4 B. 5 C. 6 D. 3

• Speed of boat in still water = 15 km/hr
  
  Speed of the stream = x
  
  Hence, speed of the boat going downstream is (15 + x) km/hr and the speed of the boat going upstream is (15-x) km/hr.
  
  Time = Distance/Speed
  
  Time going downstream = 27/(15+x)
  
  Time coming back upstream = 27/(15-x)
  
  Total time = (27/(15+x)) + (27/(15-x))
  
  3.45 = [27(15-x) + 27(15+x)] / [(15+x)(15-x)]
  
  3.45 = [810] / [225-(x^2)]
  
  776.5 - 3.45(x^2) = 810
  
  3.45(x^2) = 33.75
  
  x^2 = 9
  
  x = 3
  
  Therefore, the speed of the stream is 3 km/hr.
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