Alex and Billie have to empty all of the bookshelves in the library, and move the books into storage.
Alex, on her own, can empty the bookshelves in 18 hours.
Billie, on her own, can also empty the bookshelves in 18 hours.
Unfortunately, Drew likes to move the books from storage back onto the bookshelves, and she can refill the bookshelves in 36 hours.
How long will it take the three of them, working at the same time, to empty the bookshelves?
Puzzle Copyright © Kevin Stone
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Answer
12 hours.
Reasoning
In 36 hours, Alex could empty 2 lots of bookshelves, Billie could also empty 2 lots of bookshelves, and Drew could refill 1 lot of bookshelves.
So, 2 + 2 – 1 = 3 lots of bookshelves in 36 hours. Which is 12 hours for one lot of bookshelves.
Alternative Reasoning
Alex and Billie can each empty 1/18 of the bookshelves in 1 hour, but Drew will refill 1/36 of the bookshelves in one hour.
So in one hour we have 1/18 + 1/18 – 1/36 = 1/12.
So it would take 12 hours to empty the bookshelves.
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