Quantitative Aptitude :: Pipes & Cistern - Discussion

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Explanation:
Tank part filled by Pipe A in 1 hour = 1/12
similarly Tank part emptied by pipe B in 1 hour = 1/36
or Tank part emptied by pipe B in 15 min = 15 * 1/60*36 = 1/144

From 8 a.m. to 9 a.m. tank part filled = 1/12 => 11/12 of the tank is empty
From 9 a.m. to 10 a.m. tank part will be filled = (1/12 + 1/144) = 11/144 => 1 hour 11/144 of the tank gets filled
11/12 part of the tank would take = 11/12 x 144/11 = 12 hours
Total time required is 1 + 12 = 13 hours
=> Tank would be full @ 8 a.m. + 13 = 9 p.m.