Two pipes A and B can fill a tank in 10 and 20 minutes respectively.If both pipes are opened simultaneously,after how many minutes should pipe B be closed so that the tank is full in 8 minutes?
A. 3 min
B. 4 min
C. 2 min
D.5 min

• B. 4 min
  
  If after 'x' min. B is closed, then A & B are opened for 'x' min. and
  only A will be opened for (8-x) min.
  1 min. filling of A & B= (1/10)+(1/20)=3/20
  (3x/20)+(8-x)/10 =1, On solving x=4
  So B should be closed after 4 minutes.
• 10 years agoHelpfull: Yes(2) No(0)
• B. 4 min
  a and b can fill in 1 min= 1/10+1/20=3/20
  by checking the options:
  in 4min, while both are open= 12/20
  rest 4 min only a is open= 4/10
  12/20+4/10=1
  therefore, 4 mins
• 10 years agoHelpfull: Yes(1) No(0)
• answer is 4min.
  a and b can fill in 1 min= 1/10+1/20=3/20
  by checking the options:
  in 4min, while both are open= 12/20
  rest 4 min only a is open= 4/10
  12/20+4/10=1
  so the answer is 4min
• 10 years agoHelpfull: Yes(0) No(0)
• A's work in 8 min = 8/10
  so remaining work = 1 - 8/10 = 1/5
  bcoz B fill the tank in 20 minutes
  so time taken by B to fill 1/5 tank = 20 *(1/5) = 4 min
• 10 years agoHelpfull: Yes(0) No(0)