.one pipe fills in 4 hrs and another in 5hrs when they both work alternately how much time will be taken to fill the tank.

• in 1 hr first pipe will fill=1/4th of the tank
  and
  in 1 hr second pipe will fill=1/5 of the tank
  as these two pipes works alternately so in 2 hrs the tank will fill up=1/4+1/5 =9/20
  so in 2*2 or 4 hrs tank will fill up by 9/20*2=9/10
  so left part =1-9/10=1/10
  then first pipe will open,
  it can fill 1/4 in 1 hr
  so 1/10 will fill by it by =(1/10)/(1/4)=2/5 hr=24 minute
  so total time= 4hr 24 minute
• 13 years agoHelpfull: Yes(33) No(1)
• 4 hrs 24 mins
  
  In 2 hrs , 9/20 tank is filled.
  In 4 hrs , 18/20 tank is filled and 1/10 tank is left.
  
  1/10 tank is filled by first pipe in (1/10)*4=2/5 hrs or 24 mins.
  
  so total 4 hrs 24 mins.
• 13 years agoHelpfull: Yes(30) No(11)
• 1/4+1/5=9/20
  it takes 20/9 hrs to fill the tank
• 13 years agoHelpfull: Yes(6) No(16)
• Assume that total water in tank is 20unit (LCM of 5 & 4)
  First pipe fills in 1 hr 5unit
  Second pipe fills in 1 hr 4unit
  in 4 hrs water filled 18unit
  left remaining 2unit
  time taken by first pipe 2/5=24mint
  
  total time 4hr 24 minute
• 13 years agoHelpfull: Yes(4) No(1)