A Cistern is filled by pipe A in 9 hrs and the full Cistern can be leaked out by an exhaust pipe B in 10 hrs. If both the pipes are opened in what time the Cistern is full?
Work down by the inlet in 1 hr = 1 /9
Work done by outlet in 1 hr 1/10
Net part filled in 1 hr = 1/9 - 1/10 = 1/90
The Cistern will be full in 90 hrs.
Hint: Take the reciprocals.
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