Filling pipe, if opened alone, takes 5 min. to fill a cistern. suddenly, during the course of filling, the waste pipe (which is of similar size and flow as of fill pipe) is opened for 2 min., then the cistern will be filled in.
[A] 15 min
[C] 8 min
[B] 12 min
[D] 7 min

• fill rate =1/5 min
  and as outlet is of same size and flow>> empty rate = 1/5
  total rate for 2 minutes when both pipes are working =1/5-1/5=0>>
  >>> for 2 minutes no part will be filled and nor will be emptied
  so nw 2 minutes are wasted hence 5+2 = 7 minutes answer
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