A boat goes 24 km upstream and 28 km downstream in 6 hours. It goes 30 km upstream and 21 km downstream in 6 hours and 30 minutes. The speed of the stream is
a. 10 km/h
b. 5 km/h
c. 4 km/h
d. 6 km/h
e. None of these

• c. 4 km/h
  
  If B is speed of boat in still water and s is speed of stream, then
  24/ (B-S) + 28/(B+S) =6
  and
  30/ (B-S) + 21/(B+S) =13/2
  
  solving these eqns, we get
  speed of stream = 4 km/hr
  and speed of boat in still water = 10 kmph
• 13 years agoHelpfull: Yes(2) No(4)
• ans. 4 km/h
  24/x+28/y=6
  30/x+21/y=13/2
  multiplying both eq by 5 and 4 resp.
  120/x+140/y=30
  120/x+84/y=26
  solving this equation we get
  y(downstream)=14
  x(upstream) =6
  so,
  speed of current = 1/2*(14-6)
  =4 km/h --> answer
• 10 years agoHelpfull: Yes(1) No(0)
• if speed of the boat be x and stream speed be y
  
  from the given question,
  
  24/(x-y) + 28/(x+y) = 6
  30/(x-y) + 21/(x+y) = 6.5
  
  Taking 1/(x-y) as X and 1/(x+y) as Y
  24X + 28Y = 6
  30X + 21Y = 6.5
  
  solving for X and Y==>
  X = 1/6 and Y = 1/14
  
  so x-y = 6 and x+y = 14
  hence
  x = 10 kmph and y = 4kmph
• 10 years agoHelpfull: Yes(0) No(0)
• c. 4km/hr
  
  let the speed of the boat be X and speed of the Stream be Y.
  
  from the above details
  
  24/(x-y)+28/(x+y)=6
  
  30/(x-y)+21/(x+y)=6.30
  
  take 1/(x-y) as X and 1/(x+y) as Y
  
  24X+28Y=6 ----(1)
  
  30X+21Y=6.30 ----(2)
  
  
  solve eqn (1)&(2)
  we'll get
  X=1/6 & Y=1/14
  
  so X-Y = 6 and X+Y = 14
  
  X = 10 kmph and Y = 4kmph.
  
  so boat speed = Y= 4kmph
• 5 years agoHelpfull: Yes(0) No(0)