Two fill pipes A and B can fill a tank in 30 and 20 hours respectively. Pipe B alone is kept open for half the time and both pipes are kept open for the remaining time. In how many hours the tank will be completely full?
[A] 12 h
[C] 14 h
[B] 13 h
[D] 15 h

• Pipe A 1 hr work=1/30
  Pipe B 1 hr work=1/20
  let total time required to fill the tank =xhrs
  since pipe B is open for half hr so work done by pipe B in x/2 hrs =x/40hrs
  and work done by pipe (A+B) in remaining x/2 hrs=x/24hrs
  so total work done is =x/40+x/24
  1=x/40+x/24
  =>1=8x/120
  =>120=8x
  so x=15hrs
• 11 years agoHelpfull: Yes(16) No(0)
• let x will be completion time
  
  so, x/2*(1/20) + x/2*(1/30 + 1/20)=1
  8x=120
  x=120/8
  x=15 ans
  
• 10 years agoHelpfull: Yes(7) No(0)
• Easy
  a->30 & b->20
  take LCM TO BOTH is 60
  then a=60/30=2 & b=60/20=3
  take time as "x"hrs
  is x/2(b)+x/2(a+b)=60
  3x/2+5x/2=60 =>8x=120 =>x=15hrs (D)
  
  
• 10 years agoHelpfull: Yes(3) No(0)