From the book: "Area: A swimming pool is 20 feet wide and 40 feet long. If it is surrounded by square tiles, each of which is 1 foot by 1 foot, how many tiles are there surrounding the pool?"
Although this question is posed in the chapter on area and volume, it appears in a section named "Applying the Concepts," which is not above questions that require concepts from previous chapters to be used for solving problems.
If the tiles truly do surround the pool (or rectangle), then it seems like the problem is really asking for the perimeter. Also, I think this leaves a tile at each corner of the pool, to cover the outside corners.
Or do you think the tiles should be considered as inside the corners of the pool?