Question

pipe A can fill a Cistern in 5 hours and pipe B in 10 hours. both the pipes are opened together and after 2 hours pipe A is closed, how much time will the pipe B take to fill up the remaining part of the tank??
answer is 6 hours but how??
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Answer 1

Answer:

ans should be 4 hours to fill the remaining tank by b

and for filling full tank ans is 6