Two pipes a and b can fill a tank in 15min and 20 min respectively. both the pipes are opended together but after 4 min.pipe a is turned off. what is the total time required to fill the tank

• a's 1 min work = 1/15
  b's 1 min work = 1/20
  
  a+b's 1 min work = 1/15 + 1/20 = 35/ 300 = 7/60
  a+b's 4 min work = 4 * 7/60 = 7/15
  
  Remaining tank yet to be filled = 1-7/15 = 8/15
  
  B ll fill
  1 min ---- 1/20
  x --------8/15
  so 8/15*20 = 32/3 min
  
  So totally 4 + 32/3 = 44/3 = 14 2/3 mins ===> 14mins 40 sec
  
  Ans : 14 min 40 secs
• 10 years agoHelpfull: Yes(6) No(1)
• a's 1 min work will be 4
  b's 1 min work will be 3
  Total work will be 60
  Now a+b work for 4 min so total work done = 7*4=28
  so work left=60-28==>32
  Now time taken by b to complete remaining work=32/3
  which is 10 min 40 sec and previouslly they worked for 4 min
  So total time will be [14min 40sec]
• 10 years agoHelpfull: Yes(1) No(0)
• As both work for 4mins,work done by both for 4mins is 4(1/15 + 1/20)=28/60
  Let X be the remaining time
  only b worked for Xmin = (1/20)X
  so, 28/60 + X/20 = 1
  X/20=32/60
  X=44/3 min = 14 min.40 secs.
  
• 10 years agoHelpfull: Yes(0) No(0)