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      Question

      A boat can go 15 km upstream and 21 km downstream in 2

      hours. If the speed of the stream is 6km/h, then the time taken (in hours) by the boat to go 21 km upstream and 15 km downstream is:
      A 3.7hours Correct Answer Incorrect Answer
      B 2.3hours Correct Answer Incorrect Answer
      C 1.6hours Correct Answer Incorrect Answer
      D 4.3hours Correct Answer Incorrect Answer

      Solution

      Let the speed of boat in still water =B Let the speed of stream in still water =S Speed of boat in downstream =B+S Speed of boat in upstream =B-S B+S=21/2=10.5km/h B-S=15/2=7.5km/h Solving after – B=9km/h and S=1.5km/h ATQ- S =1.5 =6km/h B=9=36km/h Now total taken time = (21/B-S) +(15/B+S) = (21/30) +(15/42) =148/40=37/10 =3(7/10) hours

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