Quantitative Aptitude :: Pipes & Cistern - Discussion

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Explanation:
L.C.M of 4hr and 5hr will be 20units. (tank size)
Given. pipe A and B fill a tank in 4 hrs and 5 hrs res.
So, A can fill 5u in 1hr and B can fill 4u in 1hr.
(5u+4u)+(5u+4u)=18u in 4hr (alternatively opend)
Remaining 2u will be filled by pipe A
5u will take=60mins
Hence 1u =60/5min=12mins
Hence 2u will take 12*2=24mins.
Total time required = 4 hrs. + 24 min.