Puzzle Details

James and Steve were busily emptying their book shelves from one room to another.

James on his own could have cleared the room in 18 hours.

Steve also would take 18 hours.

Unfortunately, John was moving the books back into the first room from the second.

He could fill the first room in 36 hours.

How long will it take the three of them to complete the task?

In 36 hours James can clear 2 rooms, Steve can also clear 2 rooms, whilst John would fill 1 room. So 2 + 2 - 1 = 3 rooms in 36 hours. Which is 12 hours for a single room.

Alternatively:

James and Steve each clear 1/18 of the room in 1 hour, but John is filling 1/36 of the room in one hour. So in one hour we have 1/18 + 1/18 - 1/36 = 1/12. So it takes 12 hours to complete the task.

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