Mathematics
NULL
Medium
Question
A cistern can be filled by a tap in 4 hours and emptied by another in 9 hours. If both are opened together, after how long will the cistern get filled?
A
4.5 hrs
B
5 hrs
C
6.5 hrs
D
7.2 hrs
✓ Official correct answer
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A
4.5 hrs
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B
5 hrs
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C
6.5 hrs
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D
7.2 hrs
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Explanation
Net rate = 1/4 − 1/9 = 5/36. Time = 36/5 = 7.2 hours.