1. I've attached a sketch of the described swimming pool.
2. . Thus:
3. The area including the decks is:
. Now plug in the value of l:
4. . Expand the RHS of this equation:
5. This function has an extreme value (maximum or minimum, you are interested in the minimum) if the first derivative equals zero:
. Solve for w:
I've got: w = 30. Plug in this value to calculate l. I've got l = 60.