Pipe A can fill an empty tank in 5 hours while Pipe B can empty the full tank in 6 hours. If both are opened in the empty tank, time taken to fill it up completely is __________.
A
30hours
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B
20hours
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C
10hours
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D
40hours
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Solution
The correct option is A30hours Let total size of tank be 1 unit
A's one hour water contribution=1/5 units
B's one hour water contribution=−1/6 units ( minus sign for emptyting tank)
(A+B)'s one hour water output= 1/5−1/6=1/30
Time needed by (A+B) to fill whole tank= 11/30 hours=30 hours