Two pipes A and B can fill a cistern in 371⁄2 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if pipe B is turned off after:
A. 5 min
B. 9 min
C. 10 min
D. 15 min

• 9 mins.
  let B turned off after x min,then
  part fille by A+B in x min + part filled by A in 30-x min =1
  x(2/75+1/45)+(30-x)*2/75=1
  x=9
• 10 years agoHelpfull: Yes(23) No(12)
• A filled part in 1 min=1/75/2=2/75
  B=1/45
  B will be on for x min
  30/(75/2)+x/45=1
  x=9 min
• 10 years agoHelpfull: Yes(11) No(0)
• first tell me is it (371)/2 or 3 71/2 or 37 1/2 .....????????
• 9 years agoHelpfull: Yes(9) No(0)
• formulae is t2(1-x/t1) here t1 is first time t2 is second time
  x is cistern filled in 30 minutes
  so 45(1-30*2/75)=9
• 9 years agoHelpfull: Yes(1) No(1)
• let first pipe be opened for 30 min then
  30*2/75+x/45=1
  x/45=1-60/75
  x/45=15/74
  x/45=1/5
  x=45/5
  x=9 min
  ans is 9 min
• 9 years agoHelpfull: Yes(0) No(0)