The area of the whole pool is (10-ft x 40-ft) = 400 square feet.
If they divide it exactly in half, the area of each half is 200 square feet.
They'll also neatly succeed in accomplishing the kind of thing that people
who don't get along frequently accomplish . . . the layout of the pool will be
so awkward and uncomfortable that neither sister will be able to use it, and
they'll just have to stay inside and take naps or watch TV until they make up.