Four pipes can fill a reservoir in 15 20 30 and 60 hours respectively The first one was opened at 6 ...........

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Four pipes can fill a reservoir in 15, 20, 30 and 60 hours respectively. The first one was opened at 6 AM, second at 7 AM, third at 8 AM and the fourth at 9 AM. When will the reservoir be filled ?

11 AM