Answer:
24 - x would mean the air, or empty space in the bottle not being used by the water in the bottle.
Question:
"Create an equation that shows how much space is not taken up by water in a 24 once water bottle if it currently has x ounces of water."
OR
"A standing garden bed has x cubic feet in the box. It can hold 24 cubic feet, counting from the bottom to very top. How much empty space is there from the top of the dirt in the box to the very top of the box?"