A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled ?
Net part filled in 1 hour = (1/4 - 1/9)
Therefore,the cistern will be filled in 36/5 hrs i.e, 7.2 hrs.
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