A river runs at 4 km/hr. if the time taken by a man to row is boat upstream is 3 as the time taken by him to row it downstream then find the speed of the boat in still water.

• If the speed upstream is x km/hr and the speed downstream is 3x km/hr, then:
  Speed in still water = (1/2)*(x+3x) km/hr.
  
  Rate of stream = (1/2)*(3x-x) km/hr.
  So 4= (1/2)*2x; => x= 4.
  
  Thus, speed in still water= (1/2)*(4+12) => 16/2 => 8.
  So, Speed of boat in still water is 8 km/hr.
  
• 10 years agoHelpfull: Yes(28) No(1)
• Speed(upstream)=Speed(Boat)- Speed(River)
  Speed(downstream)=Speed(Boat)+ Speed(River)
  
  Given is if x is speed at downstream then 3x is speed at upstream
  speed(River)=4
  
  Substitute the values in Above eqn.
  x=Speed(Boat)+4
  3x=Speed(Boat)-4
  
  Multiply firs eqn by 3 and sub both.
  
  speed(boat)=8kmph.
  
• 10 years agoHelpfull: Yes(11) No(2)
• Time(Upstream)=3T(Downstream)
  d/x-y=3(d/x+y)
  x+y=3x-3y
  4y=2x
  x=2y
  x=8Kmph
  
• 10 years agoHelpfull: Yes(6) No(2)
• sry its 3(x-4)=x+4
• 10 years agoHelpfull: Yes(4) No(0)
• Now here let distance is same (Not mentioned) & time is inversely varies as speed=distane/time,
  so speed so solve in the following way
  let the rate in still wtr be x kmph,
  upstream x+4, dwnstream x-4,
  
  comparing speeds 3(x-4) = (x+3)
  3+y=6-3y
  y=1.5 is the correct answer
  
  
• 10 years agoHelpfull: Yes(3) No(6)
• let the rate in still wtr be x kmph,
  upstream x+4, dwnstream x-4, comparing speeds (x-4) = 3(x+3)
  gives 8kmph
• 10 years agoHelpfull: Yes(2) No(0)
• plz,explain clearly @ RAVITEJA
• 10 years agoHelpfull: Yes(1) No(0)
• Assume that he moves 4 km downstream in x hours
  
  Then, speed downstream = distance time= 4x km/hr
  
  Given that he can row 4 km with the stream in the same time as 3 km against the stream
  
  i.e., speed upstream = 3/4 of speed downstream
  speed upstream = 3x km/hr
  
  He rows to a place 48 km distant and come back in 14 hours
  
  =>48/(4x)+48/(3x)=14
  =>12/x+16/x=14
  =>6x+8x=7
  =>14x=7
  =>x=1/2
  Hence, speed downstream = 4x = 4(1/2) = 8 km/hr
  speed upstream = 3x=3(1/2) = 6 km/hr
• 10 years agoHelpfull: Yes(1) No(0)
• still water boat speed 8kmph
  
• 10 years agoHelpfull: Yes(0) No(0)
• 8
  bec. put t=3t u will get d answer
  
• 10 years agoHelpfull: Yes(0) No(0)
• azhar shahid.. why u r taken 1/2 * (3x-x) = 4 he z not mentioned dat it is a upstream.. plz explain clearly..
• 10 years agoHelpfull: Yes(0) No(2)
• speed of water = 4kmph
  upstream speed= 3kmph
  then downstream will be
  speed of water= (downstream speed- upstream speed)/2
  4=DS-3/2
  DS-3=8
  Ds=11
  so speed in still water will be=Ds-SOW
  = 11-4
  =7
  or speed in still water will be= US+SOW
  =3+4
  =7 ans.
  
• 10 years agoHelpfull: Yes(0) No(1)
• speed of current=4 km/hr time taken upstream=3t and time taken downstream=t
  speed upstream=d/3t and speed downstream=d/t
  speed of current=1/2(d-u); 4=1/2(d/t +d/3t) so 12=d/t
  so upstream speed=12/3=4 km/hr downstream=12 km/hr
  now finding speed of boat=1/2(d+u)=>1/2*16=>8 km/hr (ans)
• 7 years agoHelpfull: Yes(0) No(0)
• Let Speed of Boat = B km/hr and Distance = x km
  Speed of River (R) = 4 km/hr
  Speed Upstream = (B+R) km/h
  Speed Downstream = (B-R) km/h
  
  Now,
  Time Upstream = 3 (Time Downstream)
  x/B-R = 3*(x/B+R) ------------------------------ [Time = Distance/Speed]
  
  x/B-R = 3x/B+R
  1/B-R = 3/B+R
  B+4 = 3B - 12 ------------------------------[since R=4 km/h]
  2B = 16
  B = 8 km/hr
• 3 years agoHelpfull: Yes(0) No(0)