Two fillings pipes can fill a cistern in 10 and 12 min. respectively and when the waste pipe is open, they can together fill it in 15 min. The waste pipe can empttyy the full cistern in?
[A] 8 min
[C] 8 4/7 min
[B] 7 min
[D] 7 1/2 min

• If x mins are taken by wasre pipe to drain the cistern, then
  
  1/15 = 1/12 +1/10 -1/x
  1/x= 1/12 +1/10 - 1/15 = (5+6-4)/60 = 7/60
  x= 60/7 mins= 8 4/7 mins.... option c
• 13 years agoHelpfull: Yes(26) No(1)
• a =1/10 ,b=1/12
  a+b=1/10+1/12=11/60
  waste pipe(x) =1/x
  a+b+x =1/15
  so
  11/60-1/15
  =7/60
  then 60/7 =>84/7 min
  
• 10 years agoHelpfull: Yes(3) No(1)
• two filling pipes rate = 1/10 + 1/12 = 11/60 cis./min
  three filling pipes rate = 1/15 cis./min
  waste pipe rate = 11/60 - 1/15 = 7/60 cis./min = 60/7 = 8 4/7 min./cis.
  
• 10 years agoHelpfull: Yes(1) No(0)